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Pultec EQP clone problem



 
 
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  #1  
Old February 2nd 21, 10:42 PM posted to rec.audio.pro
gareth magennis
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Posts: 60
Default Pultec EQP clone problem

Right, I have this Pultec EQP-1A clone that apparently was motorboating, but only when attached to a patchbay.
Turns out the manufacturer knew certain serial numbered units were prone to this and offered a mod to fix the problem.
Customer took the unit to a tech who carried out the mod as described in an email from the company.
Only now the unit is something like 12dB down in level than before.

I have removed the mod, and there is in fact about 12dB drop in level with it fitted, the customer is correct.

Original Pultec schematic he
http://www.gyraf.dk/gy_pd/pultec/pultech.gif


The mod is to wire a 100 ohm resistor in parallel with a 0.1uF capacitor directly accross the output transformer secondary.

The pultec schematic shows a 1k resistor in series with a capacitor on the output, the unit is question has a factory fitted 2k2 resistor across the output, but no cap.

I am not having much joy from the company on this matter, but it seems to me that putting 100 ohms across the output transformer is going to reduce the output level rather drastically.
This transformer is directly connected to pins 2 & 3 of the output XLR, so essentialy there is 100 ohms across pins 2 & 3.
Transformer secondary measures 113 ohms DC disconnected.

I have been trying to suggest that either this 100 ohms is incorrect, or the 100 ohms in parallel with the 0.1uF capacitor is not intended to be put across the output, but should in fact be installed somewhere else.
Maybe on the grounding circuit to prevent the motorboating under certain conditions I can't create on the bench.


Any thoughts on this? I'm getting rather frustrated.


Cheers.



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  #2  
Old February 2nd 21, 11:01 PM posted to rec.audio.pro
Don Pearce[_3_]
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Posts: 2,377
Default Pultec EQP clone problem

On Tue, 2 Feb 2021 13:42:49 -0800 (PST), gareth magennis
> wrote:

>Right, I have this Pultec EQP-1A clone that apparently was motorboating, but only when attached to a patchbay.
>Turns out the manufacturer knew certain serial numbered units were prone to this and offered a mod to fix the problem.
>Customer took the unit to a tech who carried out the mod as described in an email from the company.
>Only now the unit is something like 12dB down in level than before.
>
>I have removed the mod, and there is in fact about 12dB drop in level with it fitted, the customer is correct.
>
>Original Pultec schematic he
>http://www.gyraf.dk/gy_pd/pultec/pultech.gif
>
>
>The mod is to wire a 100 ohm resistor in parallel with a 0.1uF capacitor directly accross the output transformer secondary.
>
>The pultec schematic shows a 1k resistor in series with a capacitor on the output, the unit is question has a factory fitted 2k2 resistor across the output, but no cap.
>
>I am not having much joy from the company on this matter, but it seems to me that putting 100 ohms across the output transformer is going to reduce the output level rather drastically.
>This transformer is directly connected to pins 2 & 3 of the output XLR, so essentialy there is 100 ohms across pins 2 & 3.
>Transformer secondary measures 113 ohms DC disconnected.
>
>I have been trying to suggest that either this 100 ohms is incorrect, or the 100 ohms in parallel with the 0.1uF capacitor is not intended to be put across the output, but should in fact be installed somewhere else.
>Maybe on the grounding circuit to prevent the motorboating under certain conditions I can't create on the bench.
>
>
>Any thoughts on this? I'm getting rather frustrated.
>
>
>Cheers.
>
>

Motor boating is normally caused by taking negative feedback from the
secondary of the output transformer. It is the low frequency phase
shift as the transformer runs out of puff that causes it. But this
circuit appears to have no feedback of any kind so it is hard to see
what could motorboat. They already have a Zobel network on the output
to keep things tame at the top end.

If you are losing 12dB when you add the 100 ohms, this thing has a
seriously high output impedance (caused by the lack of feedback). I
think the approach here is to see how high a resistor value you can
get away with before motorboating starts. Then perhaps drop that by
one value and stick with that. Valves are very variable, so it is very
unlikely that every unit will need so drastic a fix as 100 ohms.

d

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  #3  
Old February 3rd 21, 12:13 AM posted to rec.audio.pro
gareth magennis
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Posts: 60
Default Pultec EQP clone problem

On Tuesday, 2 February 2021 at 22:01:06 UTC, Don Pearce wrote:
> On Tue, 2 Feb 2021 13:42:49 -0800 (PST), gareth magennis
> > wrote:
>
> >Right, I have this Pultec EQP-1A clone that apparently was motorboating, but only when attached to a patchbay.
> >Turns out the manufacturer knew certain serial numbered units were prone to this and offered a mod to fix the problem.
> >Customer took the unit to a tech who carried out the mod as described in an email from the company.
> >Only now the unit is something like 12dB down in level than before.
> >
> >I have removed the mod, and there is in fact about 12dB drop in level with it fitted, the customer is correct.
> >
> >Original Pultec schematic he
> >http://www.gyraf.dk/gy_pd/pultec/pultech.gif
> >
> >
> >The mod is to wire a 100 ohm resistor in parallel with a 0.1uF capacitor directly accross the output transformer secondary.
> >
> >The pultec schematic shows a 1k resistor in series with a capacitor on the output, the unit is question has a factory fitted 2k2 resistor across the output, but no cap.
> >
> >I am not having much joy from the company on this matter, but it seems to me that putting 100 ohms across the output transformer is going to reduce the output level rather drastically.
> >This transformer is directly connected to pins 2 & 3 of the output XLR, so essentialy there is 100 ohms across pins 2 & 3.
> >Transformer secondary measures 113 ohms DC disconnected.
> >
> >I have been trying to suggest that either this 100 ohms is incorrect, or the 100 ohms in parallel with the 0.1uF capacitor is not intended to be put across the output, but should in fact be installed somewhere else.
> >Maybe on the grounding circuit to prevent the motorboating under certain conditions I can't create on the bench.
> >
> >
> >Any thoughts on this? I'm getting rather frustrated.
> >
> >
> >Cheers.
> >
> >

> Motor boating is normally caused by taking negative feedback from the
> secondary of the output transformer. It is the low frequency phase
> shift as the transformer runs out of puff that causes it. But this
> circuit appears to have no feedback of any kind so it is hard to see
> what could motorboat. They already have a Zobel network on the output
> to keep things tame at the top end.
>
> If you are losing 12dB when you add the 100 ohms, this thing has a
> seriously high output impedance (caused by the lack of feedback). I
> think the approach here is to see how high a resistor value you can
> get away with before motorboating starts. Then perhaps drop that by
> one value and stick with that. Valves are very variable, so it is very
> unlikely that every unit will need so drastic a fix as 100 ohms.
>
> d
>
> --
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus



Ok, thanks.

I haven't experienced this motorboating because apparently it only happens in certain known production units, and only when connected to a patchbay.
Presumably all those long cables and earth loops are a factor.

I doubt I will be able to make it motorboat on the bench, it appears to be fine.


Also the schematic I posted is of the original Pultec, I don't have the schematic of this clone, which is not exactly the same.
They don't have a zobel network on the output, just a 2k2 resistor across the output, no capacitor.
And obviously they use different transformers so most componet values are going to be different.


I will experiment with different resistor values to see when things start to go bad.


The problem is, the customer has a pair of these, only one of which exhibited the motorboating, and thus only one had the mod done.
Now he has one unit 12 db lower than the other.
Not good.

  #4  
Old February 3rd 21, 12:13 AM posted to rec.audio.pro
[email protected]
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Posts: 80
Default Pultec EQP clone problem

gareth magennis wrote:
===================
>
> Any thoughts on this? I'm getting rather frustrated.
>


** Motorboating in audio stages that do not use NFB is caused by supply rail coupling of imposed signal from the later stages back to the first stage - usually at a sub sonic frequency.

The inter-stage coupling caps are way too large at 270nF - try 22nF instead for each.

This shifts the -3dB point up to to 7 Hz.

Increasing the PSU filter caps - by paralleling with a 47uF - will also help reduce LF coupling.


...... Phil



  #5  
Old February 3rd 21, 12:15 AM posted to rec.audio.pro
gareth magennis
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Posts: 60
Default Pultec EQP clone problem

On Tuesday, 2 February 2021 at 23:13:16 UTC, wrote:
> gareth magennis wrote:
> ===================
> >
> > Any thoughts on this? I'm getting rather frustrated.
> >

> ** Motorboating in audio stages that do not use NFB is caused by supply rail coupling of imposed signal from the later stages back to the first stage - usually at a sub sonic frequency.
>
> The inter-stage coupling caps are way too large at 270nF - try 22nF instead for each.
>
> This shifts the -3dB point up to to 7 Hz.
>
> Increasing the PSU filter caps - by paralleling with a 47uF - will also help reduce LF coupling.
>
>
> ..... Phil




Yes, customer was reporting a "tick" every couple of seconds.
  #6  
Old February 3rd 21, 12:35 AM posted to rec.audio.pro
gareth magennis
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Posts: 60
Default Pultec EQP clone problem

On Tuesday, 2 February 2021 at 23:15:30 UTC, gareth magennis wrote:
> On Tuesday, 2 February 2021 at 23:13:16 UTC, wrote:
> > gareth magennis wrote:
> > ===================
> > >
> > > Any thoughts on this? I'm getting rather frustrated.
> > >

> > ** Motorboating in audio stages that do not use NFB is caused by supply rail coupling of imposed signal from the later stages back to the first stage - usually at a sub sonic frequency.
> >
> > The inter-stage coupling caps are way too large at 270nF - try 22nF instead for each.
> >
> > This shifts the -3dB point up to to 7 Hz.
> >
> > Increasing the PSU filter caps - by paralleling with a 47uF - will also help reduce LF coupling.
> >
> >
> > ..... Phil

> Yes, customer was reporting a "tick" every couple of seconds.



How can I force this motorboating to happen on my bench, so I can deal with it?
I've not seen it yet.





  #7  
Old February 3rd 21, 01:37 AM posted to rec.audio.pro
Mike Rivers[_2_]
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Posts: 2,158
Default Pultec EQP clone problem

On 2/2/2021 6:35 PM, gareth magennis wrote:
> How can I force this motorboating to happen on my bench, so I can deal with it?
> I've not seen it yet.


If it only happens when connected to a patchbay, it's likely due to the
cable capacitance hanging across the output.

You should ask the owner with the problem a couple of questions:

1. How long are the cables between the EQ and the patchbay? If it's
going to a normalled jack, add the length of the cable between the
normalled jack of the pair to wherever it goes.

2. If it's going to a normalled jack, what's that jack going to?

or

Does it motorboat if the normalled connection is broken (stick a cable
in the "destination" patchbay jack)

You could experiment with loading the output with a capacitor. Start
with 500 pF and go up in value to about 0.01 uF to see if you can make
it take off.

I agree than 100 ohms sounds like a pretty hefty load, but you can see
what the source impedance really is by putting in a tone and measuring
the open circuit voltage at the output. Then hang resistance across it
until the voltage drops in half (6 dB if you're using a meter calibrated
in dB). The resistance value that gives you the 6 dB drop will be
representative of the output impedance of the unit.



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  #8  
Old February 3rd 21, 02:44 AM posted to rec.audio.pro
[email protected]
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Posts: 80
Default Pultec EQP clone problem

gareth magennis is soooo annoying :

==============================
> > > Any thoughts on this? I'm getting rather frustrated.


** The feeling is mutual - pal.

> > >

> > ** Motorboating in audio stages that do not use NFB is caused by supply rail coupling of imposed signal from the later stages back to the first stage - usually at a sub sonic frequency.


> Yes, customer was reporting a "tick" every couple of seconds.


** How the hell is a tick = sub sonic oscillation?

The term "motor-boating" is kinda obvious.
Got any idea what "hiss", "hum" and buzz" actually sound like ?
Know why they have those names ?


....... Phil
  #9  
Old February 3rd 21, 02:47 AM posted to rec.audio.pro
[email protected]
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Posts: 80
Default Pultec EQP clone problem

gareth magennis is super annoying :

=========================
>
> How can I force this motorboating to happen on my bench, so I can deal with it?
> I've not seen it yet.



** FFS !!!

How will you ever know if you have fixed it ?????

Rule one of troubleshooting: " Observe the Fault "



...... Phil
  #10  
Old February 3rd 21, 12:40 PM posted to rec.audio.pro
gareth magennis
external usenet poster
 
Posts: 60
Default Pultec EQP clone problem

On Wednesday, 3 February 2021 at 01:48:00 UTC, wrote:
> gareth magennis is super annoying :
>
> =========================
> >
> > How can I force this motorboating to happen on my bench, so I can deal with it?
> > I've not seen it yet.

> ** FFS !!!
>
> How will you ever know if you have fixed it ?????
>
> Rule one of troubleshooting: " Observe the Fault "
>
>
>
> ..... Phil





Ok, I think I may have got to the bottom of this.

I found this, which I believe is the issue and its solution.
https://www.gearslutz.com/board/so-m...fications.html


There, it states the mod is a 25pf in series with 100 ohms, which makes much more sense.
I fitted it and, of course, it doesn't drop the level by 12dB.

I did measure things before finding this, and the output inpedence of the circuit is around 360 ohms, in the method described by Mike.
Thanks MIke.
I also tried to measure the low frequency rolloff, which was way below 10 Hz.
Thanks, Phil.

Maybe there never was any "motorboating", this notion came from the customer in his email to the company.
I never found any, despite adding caps on the output like Mike suggested.

 




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