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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer


I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².

Each input channel includes a microphone amplifier with the following
features:

Switchable 20 dB pad on input
Balanced transformer input
Switch selectable amplifier gain
Switchable low cut filter
Channel Fader
Pan Pot
Channel deselect switch

Each input channel contains 3 tubes/valves

Each output channel includes a Master Fader and uses 2 tubes/valves with
balanced transformer output

A stereo headphone amplifier with transformer output and gain control is
provided, using 4 tubes/valves

The mixer uses a total of 26 tubes/valves

The DC heater supply and the High Voltage supply are both regulated


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Ian Thompson-Bell Ian Thompson-Bell is offline
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Default Design for a small tube/valve mixer

John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².


Excellent. Care to post a schematic?

Cheers

Ian
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Iain Churches[_2_] Iain Churches[_2_] is offline
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Default Design for a small tube/valve mixer



"John Byrns" wrote in message
...

I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².

Each input channel includes a microphone amplifier with the following
features:

Switchable 20 dB pad on input
Balanced transformer input
Switch selectable amplifier gain
Switchable low cut filter
Channel Fader
Pan Pot
Channel deselect switch

Each input channel contains 3 tubes/valves

Each output channel includes a Master Fader and uses 2 tubes/valves with
balanced transformer output

A stereo headphone amplifier with transformer output and gain control is
provided, using 4 tubes/valves

The mixer uses a total of 26 tubes/valves

The DC heater supply and the High Voltage supply are both regulated


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/


Excellent John.

How is the monitoring handled?
Do you have pre and post fade insert points? These
are vital if the user wants to patch in an aux equaliser.

Iain


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Default Design for a small tube/valve mixer


..
"Ian Thompson-Bell" wrote in message
...
John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².


Excellent. Care to post a schematic?

It might not be prudent for John to reveal his cards
at this early stage of the game..

There are others working on the same project.

Iain


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Ian Thompson-Bell Ian Thompson-Bell is offline
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Default Design for a small tube/valve mixer

Iain Churches wrote:
.
"Ian Thompson-Bell" wrote in message
...
John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².

Excellent. Care to post a schematic?

It might not be prudent for John to reveal his cards
at this early stage of the game..

There are others working on the same project.

Iain



Do I sense some competitive spirit around here??

Cheers

Ian


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Iain Churches[_2_] Iain Churches[_2_] is offline
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Default Design for a small tube/valve mixer


"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
.
"Ian Thompson-Bell" wrote in message
...
John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².
Excellent. Care to post a schematic?

It might not be prudent for John to reveal his cards
at this early stage of the game..

There are others working on the same project.

Iain


Do I sense some competitive spirit around here??


Yes of course. That's what makes it such fun:-)
You can be sure there are six or seven people soldering
up a prototype as we speak. I am one of them:-)

Iain



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Ian Thompson-Bell Ian Thompson-Bell is offline
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Default Design for a small tube/valve mixer

Iain Churches wrote:
"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
.
"Ian Thompson-Bell" wrote in message
...
John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².
Excellent. Care to post a schematic?

It might not be prudent for John to reveal his cards
at this early stage of the game..

There are others working on the same project.

Iain

Do I sense some competitive spirit around here??


Yes of course. That's what makes it such fun:-)
You can be sure there are six or seven people soldering
up a prototype as we speak. I am one of them:-)

Iain



The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter). What's yours?

I am surprised anyone is soldering up a prototype already. Good mixer
design is non-trivial, possibly even more so for an all tube design.


Cheers

Ian
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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).


I'm not convinced he's aware of the importance of such details. Or whether he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very well.


What's yours?

I am surprised anyone is soldering up a prototype already. Good mixer
design is non-trivial, possibly even more so for an all tube design.


Absolutely right.

Graham

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Ian Thompson-Bell Ian Thompson-Bell is offline
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Default Design for a small tube/valve mixer

Eeyore wrote:

Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).


I'm not convinced he's aware of the importance of such details. Or whether he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very well.


I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required. This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.

Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.

Cheers

Ian
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Iain Churches[_2_] Iain Churches[_2_] is offline
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Default Design for a small tube/valve mixer


"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
.
"Ian Thompson-Bell" wrote in message
...
John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone
mixer similar to the one requested by ³Tynan².
Excellent. Care to post a schematic?

It might not be prudent for John to reveal his cards
at this early stage of the game..

There are others working on the same project.

Iain
Do I sense some competitive spirit around here??


Yes of course. That's what makes it such fun:-)
You can be sure there are six or seven people soldering
up a prototype as we speak. I am one of them:-)

Iain



The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter). What's yours?


I am surprised anyone is soldering up a prototype already.


I am working just on the mic pre-amp module.

I have a stock of predrilled turret boards, with tube
bases fitted, so it doesn't take long to get something
singing and dancing.

Good mixer design is non-trivial, possibly even more so for an all tube
design.


I have no real interest in building a complete mixer.
If I needed such a thing I would buy and renovate a
Vortexion or Scopetronics, something like that.

Regards
Iain




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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer

In article ,
Ian Thompson-Bell wrote:

John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².


Excellent. Care to post a schematic?


There are a couple of problems with doing that right now. First my
scanner has been broken for some time and I either need to repair it, or
buy a new one. The repair should be simple if I can locate the required
material. The problem is that I left a stack of books piled on top of
the scanner for too long a time period, and this caused the thin double
sided tape that was used to attach the glass to loose its grip. To fix
it I would need to find some suitable thin double sided tape. I should
probably just bin it and buy a new one because it only interfaces with
my old computer anyway.

The second problem is that I took this design as a two-part problem, the
design of a microphone amplifier and the design of a mixer. I
concentrated on the design of the mixer section, only laying out a
concept for my own microphone amplifier design. From previous
discussions I had assumed that the idea was to paste in the schematic of
the end users favorite vintage tube microphone amplifier. I have had a
tube microphone amplifier module sitting on my desk next to my computer
for the last 8 years, so yesterday I decided to try interfacing that
with my mixer design. I immediately ran into a major problem using this
existing microphone amplifier design, and I realized that several other
existing microphone amplifier designs would also have similar problems.

As a result I am left without a suitable design for the microphone
amplifier section until I can complete my own design. I am left
agonizing over how to accomplish all my goals for the microphone
amplifier without having to make compromises that I would rather avoid.

I have also started thinking about packaging issues, which may lead to
some changes in the mixer section. I can't remember if the OP said
anything about his size and weight requirements?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer

In article i,
"Iain Churches" wrote:

"John Byrns" wrote in message
...

I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².

Each input channel includes a microphone amplifier with the following
features:

Switchable 20 dB pad on input
Balanced transformer input
Switch selectable amplifier gain
Switchable low cut filter
Channel Fader
Pan Pot
Channel deselect switch

Each input channel contains 3 tubes/valves

Each output channel includes a Master Fader and uses 2 tubes/valves with
balanced transformer output

A stereo headphone amplifier with transformer output and gain control is
provided, using 4 tubes/valves

The mixer uses a total of 26 tubes/valves

The DC heater supply and the High Voltage supply are both regulated


Excellent John.

How is the monitoring handled?


I simply have a stereo headphone amplifier with an additional gain
control connected to the output of the line amplifier, I thought this
was all the OP wanted? If it was up to me I would include an additional
stereo monitor bus with provisions for connecting the output of any
input channel to monitor bus, as well as the ability to connect the
output of the mixer, this would add complexity though.

Do you have pre and post fade insert points? These
are vital if the user wants to patch in an aux equaliser.


There are no insert points at all in my mixer design, I didn't think the
OP wanted those sorts of complications?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer

In article ,
Ian Thompson-Bell wrote:

Iain Churches wrote:
"Ian Thompson-Bell" wrote in message
...
Iain Churches wrote:
.
"Ian Thompson-Bell" wrote in message
...
John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².
Excellent. Care to post a schematic?

It might not be prudent for John to reveal his cards
at this early stage of the game..

There are others working on the same project.

Iain
Do I sense some competitive spirit around here??


Yes of course. That's what makes it such fun:-)
You can be sure there are six or seven people soldering
up a prototype as we speak. I am one of them:-)

Iain


The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter). What's yours?


The current iteration of my design has a gain of 80 dB from microphone
input to line output, 10 dB of this is in the output amplifier,
excluding makeup gain. I used this number because it was repeatedly
mentioned in the original thread, however I am not a "pro audio"
designer and don't know exactly how much gain might actually be
required. I would like enough gain to be able to use RCA ribbon mics,
or similar, for that vintage sound, is 80 dB enough for this?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer

In article ,
Ian Thompson-Bell wrote:

Eeyore wrote:

Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).


I'm not convinced he's aware of the importance of such details. Or whether
he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very well.


I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required. This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.

Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.


I wouldn't expect an inline pad to have a significant or fatal effect on
phantom power except in very rare cases, if the correct pad topology is
used, or am I missing something? On the other hand if you use the wrong
pad topology you could easily be in serious trouble.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



John Byrns wrote:

I concentrated on the design of the mixer section


Yes, that'll require a triode with AC coupled nfb from anode to grid to form
the virtual earth, the remainder being routine mix Rs, level controls etc.

That's hardly a DESIGN. It's a stock circuit.

Graham



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Rudy Rudy is offline
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Default Design for a small tube/valve mixer


"Eeyore" wrote in message
...
:
:
: John Byrns wrote:
:
: I have also started thinking about packaging issues
:
: That's about the FIRST thing I considered. The actual circuitry is not
really
: very difficult although I'd like to add a couple of nice tweaks of my own
: invention to provide low-output-Z buffering of the anodes, that'll
serously
: improve mix bus noise performance.
:
: Graham
:
Graham, that's amazing, you've been talking circuits & electronics
these past two weeks, surpassing your combined RAT history before
that ;-)
Don't tell us, it's like you're starting to fit in (niche's available
optionally.

Ok, if you say you've given packaging issues a thought,
what is your target heat load ? max weight ? size ?

Rudy

ps
did you have any luck getting EF804's, yet ?

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Default Design for a small tube/valve mixer



John Byrns wrote:

I have also started thinking about packaging issues


That's about the FIRST thing I considered. The actual circuitry is not really
very difficult although I'd like to add a couple of nice tweaks of my own
invention to provide low-output-Z buffering of the anodes, that'll serously
improve mix bus noise performance.

Graham

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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



John Byrns wrote:

"Iain Churches" wrote:

How is the monitoring handled?


I simply have a stereo headphone amplifier with an additional gain
control connected to the output of the line amplifier


And a SET stage here will mean the what the operator is monitoring IS NOT the
same signal as going to the recording equipment. As such it won't even be worth
the name as a 'monitor'.

Graham

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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).


I'm not convinced he's aware of the importance of such details. Or whether he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very well.


I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required.


It hasn't yet been mentioned but I would certainly provide it. It would be almost
unusable without same.


This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.


All very true.


Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.


No, a pad isn't a problem. The phantom power resistors still go direct to the XLR
pins 2 and 3.

Graham

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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



Iain Churches wrote:

I have no real interest in building a complete mixer.
If I needed such a thing I would buy and renovate a
Vortexion or Scopetronics, something like that.


It was in a Vortexion that I first saw a valve virtual earth configuration.

Graham



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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



Rudy wrote:

"Eeyore" wrote
: John Byrns wrote:
:
: I have also started thinking about packaging issues
:
: That's about the FIRST thing I considered. The actual circuitry is not
: really very difficult although I'd like to add a couple of nice tweaks of
my own
: invention to provide low-output-Z buffering of the anodes, that'll
: serously: improve mix bus noise performance.
:
: Graham
:
Graham, that's amazing, you've been talking circuits & electronics
these past two weeks, surpassing your combined RAT history before
that ;-)
Don't tell us, it's like you're starting to fit in (niche's available
optionally.


My interest here has always been entirely genuine. However there are some with
such entrenched and unrealistic views concerning tubes/valves that 'arguments'
over how they actually *really * perform as opposed to mythical ideas about
them have tended to obscure that.

I have no illusion that the interesting colourations they can add are genuinely
musically useful. I do however have a real problem with those who believe that
this colouration is some kind of 'fidelity' though. It's NOT. It's an
'effect'. Using suitable 'effects' isn't a bad thing though.


Ok, if you say you've given packaging issues a thought,
what is your target heat load ? max weight ? size ?


I was considering construction and presentation issues primarily. I'm toying
with a 19" rack style idea (maybe 3 u) with plug-in cassettes.

The heat won't really be that bad. Each mic amp may need only a single dual
triode such as an ECC83 or ECC88 or perhaps a couple of pentodes like the EF86
or EF804. Weight will be minimal.

Graham

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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer

In article ,
Eeyore wrote:

John Byrns wrote:

I concentrated on the design of the mixer section


Yes, that'll require a triode with AC coupled nfb from anode to grid to form
the virtual earth, the remainder being routine mix Rs, level controls etc.


There's also the buffers for the,the channel faders and pan pots, the
output amplifier, and a few other miscellaneous items.

That's hardly a DESIGN. It's a stock circuit.


Perhaps, isn't everything you design a stock circuit too, especially
with all those op-amps?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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John Byrns John Byrns is offline
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Default Design for a small tube/valve mixer

In article ,
Eeyore wrote:

John Byrns wrote:

"Iain Churches" wrote:

How is the monitoring handled?


I simply have a stereo headphone amplifier with an additional gain
control connected to the output of the line amplifier


And a SET stage here will mean the what the operator is monitoring IS NOT the
same signal as going to the recording equipment. As such it won't even be
worth
the name as a 'monitor'.


Both the line amplifier and the headphone amplifier are push pull
circuits. The microphone amplifiers are "SE", as 6 push pull microphone
amplifiers seemed a bit much for a portable mixer.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Default Design for a small tube/valve mixer

In article ,
Eeyore wrote:

Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).

I'm not convinced he's aware of the importance of such details. Or
whether he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very
well.


I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required.


It hasn't yet been mentioned but I would certainly provide it. It would be
almost
unusable without same.

This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.


All very true.

Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.


No, a pad isn't a problem. The phantom power resistors still go direct to the
XLR
pins 2 and 3.


You obviously missed the "in-line pad" part, an in-line pad definitely
has the potential to cause a problem if the wrong type of pad is used.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Eeyore Eeyore is offline
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Default Design for a small tube/valve mixer



John Byrns wrote:

Eeyore wrote:
John Byrns wrote:

I concentrated on the design of the mixer section


Yes, that'll require a triode with AC coupled nfb from anode to grid to form
the virtual earth, the remainder being routine mix Rs, level controls etc.


There's also the buffers for the,the channel faders and pan pots, the
output amplifier, and a few other miscellaneous items.


Are you claiming that you need to *design* a cathode follower for example ? Or
*design* a volume control ?

Incidentally, I have a buffer design that I reckon's the dog's doo-dahs and would
suit this down to the ground but it's not purist thermionics sadly. However its
transparency might just allow its use.


That's hardly a DESIGN. It's a stock circuit.


Perhaps, isn't everything you design a stock circuit too, especially
with all those op-amps?


Actually, the average mixer is mostly stock circuits. The skill is in assembling
them together to best and most efficient effect.

Then there are a few 'twiddly bits' where the designer's skill comes to the fore.
That's why there are differences.

Graham



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Eeyore Eeyore is offline
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John Byrns wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:
Eeyore wrote:
Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).

I'm not convinced he's aware of the importance of such details. Or
whether he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very
well.

I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required.


It hasn't yet been mentioned but I would certainly provide it. It would be
almost
unusable without same.

This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.


All very true.

Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.


No, a pad isn't a problem. The phantom power resistors still go direct to the
XLR pins 2 and 3.


You obviously missed the "in-line pad" part, an in-line pad definitely
has the potential to cause a problem if the wrong type of pad is used.


The pad is *inline* with the signal path, NOT the phantom power.

Graham

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John Byrns wrote:

Both the line amplifier and the headphone amplifier are push pull
circuits.


What on earth for ? Do you LIKE crossover distortion ?

Graham

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In article ,
Eeyore wrote:

John Byrns wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:
Eeyore wrote:
Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain
(or
overall gain for that matter).

I'm not convinced he's aware of the importance of such details. Or
whether he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very
well.

I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required.

It hasn't yet been mentioned but I would certainly provide it. It would
be
almost
unusable without same.

This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages
with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.

All very true.

Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the
first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.

No, a pad isn't a problem. The phantom power resistors still go direct to
the
XLR pins 2 and 3.


You obviously missed the "in-line pad" part, an in-line pad definitely
has the potential to cause a problem if the wrong type of pad is used.


The pad is *inline* with the signal path, NOT the phantom power.


Can you explain how an inline pad is going to be able to avoid being in
series with the phantom power?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Default Design for a small tube/valve mixer

In article ,
Eeyore wrote:

John Byrns wrote:

Both the line amplifier and the headphone amplifier are push pull
circuits.


What on earth for ? Do you LIKE crossover distortion ?


Actually I hate crossover distortion. Where is the crossover distortion
going to come from, both the line amplifier and the headphone amplifier
operate in class A?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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Default Design for a small tube/valve mixer



John Byrns wrote:

Eeyore wrote:
John Byrns wrote:
Eeyore wrote:

No, a pad isn't a problem. The phantom power resistors still go direct to
the XLR pins 2 and 3.

You obviously missed the "in-line pad" part, an in-line pad definitely
has the potential to cause a problem if the wrong type of pad is used.


The pad is *inline* with the signal path, NOT the phantom power.


Can you explain how an inline pad is going to be able to avoid being in
series with the phantom power?


HOLY ****.

You're dumb as they come for sure.

The phantom power NEVER flows through the pad (or it shouldn't at least). See if
you can't draw yourself a circuit with the 6k8 phnatom power Rs going to pins 2 and
3 of the XLT input connector for the mic and try and work it out for yourself will
you ?

Graham



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Default Design for a small tube/valve mixer



is 80 dB enough for this?


Regards,

John Byrns


With the mics I am currently using, 80dB is good.My current mic pres have
66, and that is *barely* enough.
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Are you claiming that you need to *design* a cathode follower for example ? Or
*design* a volume control ?


Well, you'd need to know if the OP user wants slide pots or twist knobs.
and other such ergonomic considerations. And select such accordingly.
But that would not change anything in the schematic design.
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"Tynan AgviŠr" wrote in message
. 70...


is 80 dB enough for this?


Regards,

John Byrns


With the mics I am currently using, 80dB is good.My current mic pres have
66, and that is *barely* enough.


OK. So that's me back to the drawing board:-)
Except that my mic pre is to be plugged into the
line input of a console so 60dB is more than sufficient.

The unit that I have seen recently has
0dB output for 5mV input. That's 44dB

Iain


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"Iain Churches" wrote:

"John Byrns" wrote in message
...


Excellent John.

How is the monitoring handled?


I simply have a stereo headphone amplifier with an additional gain
control connected to the output of the line amplifier, I thought this
was all the OP wanted?


I wonder if this will be sufficient, although an addition buss does
add to the complexity.

Do you have pre and post fade insert points? These
are vital if the user wants to patch in an aux equaliser.


There are no insert points at all in my mixer design, I didn't think the
OP wanted those sorts of complications?


I was just thinking ahead. He may not want them now, but
experience tells me that it will not be long before he wants
to patch some extermal EQ or processor (even a simple limiter)
into a single channel or across the output mix buss.

A simple mixer, if it is to be of any practical use, is not
actually terribly simple.

Iain


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Default Design for a small tube/valve mixer

John Byrns wrote:
In article ,
Ian Thompson-Bell wrote:

John Byrns wrote:
I have completed a preliminary design for a 6-in 2-out microphone mixer
similar to the one requested by ³Tynan².

Excellent. Care to post a schematic?


There are a couple of problems with doing that right now. First my
scanner has been broken for some time and I either need to repair it, or
buy a new one. The repair should be simple if I can locate the required
material. The problem is that I left a stack of books piled on top of
the scanner for too long a time period, and this caused the thin double
sided tape that was used to attach the glass to loose its grip. To fix
it I would need to find some suitable thin double sided tape. I should
probably just bin it and buy a new one because it only interfaces with
my old computer anyway.


Hope you get it fixed soon. I would love to see what you have come up
with. I notice your web page is mainly radio related - is that you main
tube interst?

The second problem is that I took this design as a two-part problem, the
design of a microphone amplifier and the design of a mixer. I
concentrated on the design of the mixer section, only laying out a
concept for my own microphone amplifier design. From previous
discussions I had assumed that the idea was to paste in the schematic of
the end users favorite vintage tube microphone amplifier. I have had a
tube microphone amplifier module sitting on my desk next to my computer
for the last 8 years, so yesterday I decided to try interfacing that
with my mixer design. I immediately ran into a major problem using this
existing microphone amplifier design, and I realized that several other
existing microphone amplifier designs would also have similar problems.


Welcome to the world of mixer design.

As a result I am left without a suitable design for the microphone
amplifier section until I can complete my own design. I am left
agonizing over how to accomplish all my goals for the microphone
amplifier without having to make compromises that I would rather avoid.


Welcome to the world of mixer design.

I have also started thinking about packaging issues, which may lead to
some changes in the mixer section. I can't remember if the OP said
anything about his size and weight requirements?


This is important not only from an aethestics point of view but also
from an electronic performance one too.

Good Luck

Ian


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John Byrns wrote:
In article ,
Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter). What's yours?


The current iteration of my design has a gain of 80 dB from microphone
input to line output, 10 dB of this is in the output amplifier,
excluding makeup gain. I used this number because it was repeatedly
mentioned in the original thread, however I am not a "pro audio"
designer and don't know exactly how much gain might actually be
required. I would like enough gain to be able to use RCA ribbon mics,
or similar, for that vintage sound, is 80 dB enough for this?



I know of no pro mixer with more than 80dB gain so from that point of
view it should be enough. It will be hard to get better than 50dB S/N
with 80dB of gain so more would be rather pointless and I don't think
even the most insenstive ribbon would need more gain than that.

The big question is what method have you used to vary the gain?

Cheers

Ian
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Default Design for a small tube/valve mixer

John Byrns wrote:
In article ,
Ian Thompson-Bell wrote:

Eeyore wrote:
Ian Thompson-Bell wrote:

The one thing John did not mention in his spec was the mic pre gain (or
overall gain for that matter).
I'm not convinced he's aware of the importance of such details. Or whether
he
even knows what output levels it'll be required to deliver.

Also, a preamp circuit with a single fixed gain will not perform very well.

I must confess I had *assumed* switchable gain (say in 5 dB or 10dB
steps) would be required. This is quite a challenge in an all tube
design where there are several zeros in the open loop response and the
open loop gain is limited. The key seems to be the range of gains to be
encompassed. 0dB to 80dB would be quite a challenge, 20dB to 60dB less
so but both would need at least two separately switched gain stages with
one stage bypassed at low gains. You can get 20dB of *free* gain from
the input transformer of course, but with a 0dBu input, even with the
pad in place there is 0dBu on the grid of the first valve which in
itself would be an issue for an ECC83 biased at a volt or less. Not to
mention that with say a minimum stage gain of 20dB the signal at the
anode of the first stage tube is at +20dBu.

Provided there is little close miking with sensitive condensers then a
0dBu input is unlikely so I would suggest designing for a maximum input
of -20dBu, which with a 20dB pad available limits the input to the first
stage to -20dBu. In the unlikely event of higher levels occurring the
simplest solution would be to used a balanced in-line pad, except I
guess that screws up phantom powering.


I wouldn't expect an inline pad to have a significant or fatal effect on
phantom power except in very rare cases, if the correct pad topology is
used, or am I missing something? On the other hand if you use the wrong
pad topology you could easily be in serious trouble.



Not sure, I was simply thinking of the additional series resistance - I
have never really thought about the details of an in-line pad but if you
wanted it to still look like about a 2K load to the mic you would have a
1K in each leg with with a 222R across for a 20dB pad. I suppose
that's not too bad as the two 1Ks are in parallel as far as the phantom
power is concerned so an extra 500R in series with the 6K8 phantom feed
resistors should not hurt.

OK, that's good, set the gain to 20dB minimum and use an external
in-line pad for high SPLs.

Cheers

Ian
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Eeyore wrote:

John Byrns wrote:

I concentrated on the design of the mixer section


Yes, that'll require a triode with AC coupled nfb from anode to grid to form
the virtual earth, the remainder being routine mix Rs, level controls etc.

That's hardly a DESIGN. It's a stock circuit.

Graham


Hmmm, that assumes virtual earth mixing is the way to go for tubes. A
typical triode will have a stage gain of 20 maybe 30dB so the virtual
earth will not be that good (Rfb/30 maybe) and there's not really enough
gain to make up for mix losses. Also Rfb needs to be large enough not to
significantly load the anode, so we are talking 300K or so here which
means the mix resistors will be 100K for for 10dB gain make up.


If you want to stick with VE mixing then a tubed long tailed pair and
another triode will make a simple op amp like circuit where Rfb can be
much smaller and the open loop gain is much higher making a better VE.

Or simpler still just stick a CF on the end of the triode and feed its
cathode back to the grid via a lowish Rfb.

Cheers

Ian
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Eeyore wrote:

The pad is *inline* with the signal path, NOT the phantom power.

Graham


Actually Graham I was referring to a pad external to the mixer, added
in-line with the mic cable so the pad *is* in-line with the phantom
power. This pad is additional to the 20dB pad inside the mixer which as
you rightly say in only in the signal path.

Cheers

Ian
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John Byrns wrote:

Can you explain how an inline pad is going to be able to avoid being in
series with the phantom power?



Only if it is *inside* the mic pre.

Ian
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