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Hum Measurements



 
 
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  #1  
Old September 26th 05, 01:14 AM
Joseph Meditz
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Default Hum Measurements

I've recently rebuilt the circuit board of my Champ clone that I built
a number of months for a number of reasons the biggest being an
objectionable amount of hum. 60 Hz was plainly audible plus along with
a much louder 120 Hz buzz.

Now I am very happy with the results of the modifications both in terms
of greatly reduced hum and improved tone. However, there is still a
tiny bit of audible hum. I know it is not coming from the pre amp
since its level doesn't change with the volume setting and because it
is still there when I remove the 12AX7.

On the scope I see a small signal at the speaker terminals. Removing
the speaker and connecting a 10 Ohm resistor I observed a signal of
about 2 mV p-p. It is mostly a 60 Hz with a smaller 120 Hz component
both of which are audible.

Further measurement revealed some faint higher frequency noise of about
7 mV p-p. From these measurements I conclude that in the worst case the
power delivered to the 10 Ohm load is less than 1.5 uW!

The resulting hum is very low in my opinion as I don't recall ever
hearing a quieter guitar tube amp. Nevertheless, on my next project I
will endeavor to reduce it further. I was just surprised that such a
small signal would be audible at all. Is that in line with other
builders experience?

Joe

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  #2  
Old September 26th 05, 02:36 AM
John Stewart
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Joseph Meditz wrote:

> I've recently rebuilt the circuit board of my Champ clone that I built
> a number of months for a number of reasons the biggest being an
> objectionable amount of hum.


For a full wave CT rectifier one of the transformer HT leads usually has
more resistance than the other. So a line frequency component (60 HZ in NA)
arrives at the filter. The filter is less able to attenuate the 60 HZ than
the 120 HZ in the ripple, so could arrive at your filter output fairly
large. Have a look at the HT at the filter output. One easy fix if that is
the case would be to add a small resistance in the HT lead having the least
resistance so that the rectifier will see equal sources.

If you are using the chassis for the heater returns as some do you should
change the heater leads to twisted pair. The common of the amp & it's PS
should be connected to the chassis only at the signal input connector. Be
careful with the CT lead from the HT winding to the first cap. A serious
ground loop could develop altho in that case the interference would be at
120 HZ. Cheers, JLS

> 60 Hz was plainly audible plus along with
> a much louder 120 Hz buzz.
>
> Now I am very happy with the results of the modifications both in terms
> of greatly reduced hum and improved tone. However, there is still a
> tiny bit of audible hum. I know it is not coming from the pre amp
> since its level doesn't change with the volume setting and because it
> is still there when I remove the 12AX7.
>
> On the scope I see a small signal at the speaker terminals. Removing
> the speaker and connecting a 10 Ohm resistor I observed a signal of
> about 2 mV p-p. It is mostly a 60 Hz with a smaller 120 Hz component
> both of which are audible.
>
> Further measurement revealed some faint higher frequency noise of about
> 7 mV p-p. From these measurements I conclude that in the worst case the
> power delivered to the 10 Ohm load is less than 1.5 uW!
>
> The resulting hum is very low in my opinion as I don't recall ever
> hearing a quieter guitar tube amp. Nevertheless, on my next project I
> will endeavor to reduce it further. I was just surprised that such a
> small signal would be audible at all. Is that in line with other
> builders experience?
>
> Joe


  #3  
Old September 26th 05, 04:03 AM
Phil Allison
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Default


"Joseph Meditz"
>
> I've recently rebuilt the circuit board of my Champ clone that I built
> a number of months for a number of reasons the biggest being an
> objectionable amount of hum. 60 Hz was plainly audible plus along with
> a much louder 120 Hz buzz.
>
> Now I am very happy with the results of the modifications both in terms
> of greatly reduced hum and improved tone. However, there is still a
> tiny bit of audible hum. I know it is not coming from the pre amp
> since its level doesn't change with the volume setting and because it
> is still there when I remove the 12AX7.
>
> On the scope I see a small signal at the speaker terminals. Removing
> the speaker and connecting a 10 Ohm resistor I observed a signal of
> about 2 mV p-p. It is mostly a 60 Hz with a smaller 120 Hz component
> both of which are audible.
>
> Further measurement revealed some faint higher frequency noise of about
> 7 mV p-p. From these measurements I conclude that in the worst case the
> power delivered to the 10 Ohm load is less than 1.5 uW!
>
> The resulting hum is very low in my opinion as I don't recall ever
> hearing a quieter guitar tube amp. Nevertheless, on my next project I
> will endeavor to reduce it further. I was just surprised that such a
> small signal would be audible at all. Is that in line with other
> builders experience?
>



** A power level of 1 uW is 60 dB below a power of 1 watt.

The SPL with ear held close to an efficient, 12 inch speaker for 1 watt
could be 120 dB.

120 - 60 = 60 .




........... Phil


  #4  
Old September 26th 05, 04:04 AM
Joseph Meditz
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Posts: n/a
Default

Hi John,

>For a full wave CT rectifier one of the transformer HT leads usually has
>more resistance than the other. So a line frequency component (60 HZ in NA)
>arrives at the filter. The filter is less able to attenuate the 60 HZ than
>the 120 HZ in the ripple, so could arrive at your filter output fairly
>large. Have a look at the HT at the filter output. One easy fix if that is
>the case would be to add a small resistance in the HT lead having the least
>resistance so that the rectifier will see equal sources.


As I recall it, the ripple on the B+ looked fairly uniform. I don't
recall every other bump a different size. But I'll keep this in mind
the next time I do some measurements. Also, the AC voltage on both
sides of the center tap were equal on the DMM.

However, the 120 Hz ripple is a bit higher than the simulation results,
0.2 Vp-p vs. .12 Vp-p.


>If you are using the chassis for the heater returns as some do you should
>change the heater leads to twisted pair.


I would never use the chassis to carry as a current carrying return. I
do have twisted the heater wires and have positioned them close to the
chassis. Btw, on my next project just for fun I'm going to feed the
heaters with shielded wire.

>The common of the amp & it's PS
>should be connected to the chassis only at the signal input connector.


That's the way I have it.

> Be
>careful with the CT lead from the HT winding to the first cap. A serious
>ground loop could develop altho in that case the interference would be at
>120 HZ. Cheers, JLS


Thanks for your thoughts on this John.

Joe



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  #5  
Old September 26th 05, 05:13 PM
Joseph Meditz
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Default

>** A power level of 1 uW is 60 dB below a power of 1 watt.
>
>The SPL with ear held close to an efficient, 12 inch speaker for 1 watt
>could be 120 dB.
>
>
>120 - 60 = 60 .
>
>
>.......... Phil


So it's well within the range of audibility. Thanks Phil.

Joe

  #6  
Old September 29th 05, 02:37 AM
John Stewart
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Default

Joseph Meditz wrote:

> I've recently rebuilt the circuit board of my Champ clone that I built
> a number of months for a number of reasons the biggest being an
> objectionable amount of hum. 60 Hz was plainly audible plus along with
> a much louder 120 Hz buzz.
>
> Now I am very happy with the results of the modifications both in terms
> of greatly reduced hum and improved tone. However, there is still a
> tiny bit of audible hum. I know it is not coming from the pre amp
> since its level doesn't change with the volume setting and because it
> is still there when I remove the 12AX7.


When you remove the 12AX7 you are disabling the NFB which plays an
important part in keeping the hum in the circuit at a reasonable level. As
well, pulling the 12AX7 allows whatever PS Hum voltage is still at the 3rd
filter cap to be applied to the 6V6 G1 & be amplified. While the 12AX7 is
plugged in it does quite a good job at attenuating that same hum voltage
since it's plate forms a more effective voltage divider with the 100K plate
load then the following 6V6 G1 220K resistor alone. All not obvious without
a careful look at the circuit.

Cheers, John Stewart

> On the scope I see a small signal at the speaker terminals. Removing
> the speaker and connecting a 10 Ohm resistor I observed a signal of
> about 2 mV p-p. It is mostly a 60 Hz with a smaller 120 Hz component
> both of which are audible.
>
> Further measurement revealed some faint higher frequency noise of about
> 7 mV p-p. From these measurements I conclude that in the worst case the
> power delivered to the 10 Ohm load is less than 1.5 uW!
>
> The resulting hum is very low in my opinion as I don't recall ever
> hearing a quieter guitar tube amp. Nevertheless, on my next project I
> will endeavor to reduce it further. I was just surprised that such a
> small signal would be audible at all. Is that in line with other
> builders experience?
>
> Joe


 




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