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How can I reduce the gain in my ASL Wave AV-8?

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mike_decock

(A)

I like the sound of my Waves, but I just can't get rid of the transformer hum (not a ground loop or grounding issue).
I with thinking if I reduced the gain on the amp, the hum would be reduced and it would work better with my preamp which drives it as loud as I need it at 8 O'Clock (barely on and just over it's noise floor).
-Mike...

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Re: How can I reduce the gain in my ASL Wave AV-8? - zarniwoop 11:34:21 09/23/02 (0)
Just a quick idea that you probably have already tried. Remove the cathode resistor bypass capacitor on the first triode. Gives some local feedback and lowers the gain.
You could also increase the global feedback to lower the gain and reduce hum. If mains hum is really the problem then maybe you could replace the smoothing capacitor with a larger one for some improvement.
Re: How can I reduce the gain in my ASL Wave AV-8? - Tiny Daddy 09:16:38 09/23/02 (5)
You are not the first to post this humm problem. Apparently WAVE was not careful with the grounds - see previous posts.
If you want to reduce the gain it would be better done without using an input attenuator (for noise reasons). Is a schematic posted somewhere?
- Here's a schematic - mike_decock 09:32:53 09/23/02 (4)
  I've dug through the archives for the hum problem and last night I went as far as running the ground sides of R14 and R15 to the chassis ground instead of the star. One inmate said that this solved his hum problem but it didn't work for me.
  I think that reducing the gain would be preferable to attenuating the input.
  -Mike...
  - Wave AV-8 Info
  - Re: Here's a schematic - Tiny Daddy 17:04:41 09/23/02 (0)
    If you remove C6 and C7 output cathode bypass caps this should also reduce the gain.
  - Re: Here's a schematic - Saurav 15:03:01 09/23/02 (0)
    > I've dug through the archives for the hum problem and last night I went as far as running the ground sides of R14 and R15 to the chassis ground instead of the star. One inmate said that this solved his hum problem but it didn't work for me.
    I tried that too, didn't seem to do much. One thing that did work for me was replacing the Sevtlanas with the original Eis. Those tubes seem to hum much less in my system. Of course, I've hacked the amps up quite a bit, so the wiring dress is fairly different from what it was originally, and probably quite sub-optimal. Also, my Svetlanas are old Russian stock, not the new production ones that others seem to be using. In any case, the hum went from being audible 6' away from the speaker, to inaudible at 1'. Before I tried the tube change, I was almost going crazy moving wires around, and trying all kinds of stuff.
  - Re: Here's a schematic - Tiny Daddy 10:17:35 09/23/02 (1)
    OK I see the schematic and it looks like all that extra gain is coming from R4 which is a whopping 390K.
    So the gain may be adjusted and the operating point maintained by adjusting R4 and R5 while keeping their sum equal to 400K. For instance change R4 to 100K and R5 to 300K. The current is small so use 1/2 watt resistors.
    - THANKS! - mike_decock 07:39:53 09/24/02 (0)
      I'll try out your suggestion and report back the results.
      -Mike...
Re: How can I reduce the gain in my ASL Wave AV-8? - Saurav 09:04:17 09/23/02 (1)
You could play with the input sensitivity quite easily, but I don't think that will help you with hum generated by the Waves. When I was using a Foreplay with the Waves, I changed the amps so that they went to 1V input sensitivity, instead of 500mV. That gave me a little more useful range on the Foreplay's volume control, and hopefully brought it a little more into its "sweet spot", i.e. further away from its noise floor.
If you're sure that the hum is from the amps, I don't know what you could do. However, if some of the hum is upstream noise that is being amplified, then you can change that easily. This will reduce "gain" in the sense of total input-to-output gain, but not the gain of the amplifier stage.
Anyway, here's what I did - I built a resistor voltage divider. The input impedance of the Waves is 100K, which is done by a resistor (obviously). One end of this resistor goes to ground, the other end goes to the RCA center pin and the tube input. I disconnected the 100K-RCA wire (this is the black shielded input wire) from the RCA. Then I put in 2 50K resistors in series between the RCA center pin and ground, and reattached the black cable to the mid-point of these two 50K resistors. So it looks something like this:
```
  
 Before:  o----------- 
             | 
             R (100K) 
             | 
             V 
 After: 
 o-- 
    | 
    R1 (50K) 
    | 
    o----------- 
    |           | 
    R2 (50K)    R (100K) 
    |           | 
    V           V 
  
```
Now the input voltage to the actual amplification stage is half of what it used to be, since you're picking it up between the two 50K resistors. The new input impedance is 50 + (50 || 100), which is roughly 83K, close enough to what it was. If you want, you can pick the values of the two resistors to get any input impedance you want (meaning you can make it exactly 100K), and any voltage division ratio you want. 2 unknowns, 2 equations, pretty easy to solve:
Voltage ratio = R2 / (R1 + R2)
Input impedance = R1 + (R2 * 100 / (R2 + 100)) [i.e. R1 + (R2||100)]
Hope that makes sense :)
Saurav
- Thanks - mike_decock 09:37:22 09/23/02 (0)
  I may try that if I can't figure out how to reduce the gain.
  Had I been into this stuff when I went to ITT I would probably have retained more of my knowledge but it's coming back to me :).
  -Mike..

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