Tone control for my Leach amp. - diyAudio
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Old 24th February 2005, 01:35 AM   #1
murat is offline murat  United States
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Default Tone control for my Leach amp.


Finally my Leach amp is up and running. Although the bass sounds somewhat weak I liked it a lot.

Considering I have lots of old stuff recorded in the third world with very low technical capabilities, I need a little bass-treble control to keep them listenable.

I am planning to adopt only the tone control part of Rod Elliot's P-97 and use only a pot for volume and no balance. If the idea work, I will end up with only one opamp on the signal line instead of three. I am aware of some potential problems but my knowledge is nowhere close to solve them. So, some questions will be boring for most of you:

0) I am open to any other minimalist tone control circuit suggestion.

1) Rod says: "The input stage is configured as shown with a gain of 2 times (6dB), and also acts as a buffer for the tone control circuit. Let's suppose I can live without this extra 6dB. What does the buffer do and what happens if I skip this stage?

2) If it is OK to skip the input stage, then, should I keep any components of this stage? There seem to be an input filter: R 101, 102, 103 and C100. How about R 111 before the ton control part?

3) The dangerous (?) part: "The final stage is inverting - this is to correct for the inversion in the tone controls, and brings the overall phase back to normal". Do I have to invert it back?

4) The same as the 2nd question for the balance and volume stage. If it is OK to skip the second inversion, what exactly should I keep in the figure 4 except the volume pot?

5) Is OPA2134 the best for this circuit?

Thanks a lot

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Old 24th February 2005, 03:13 PM   #2
murat is offline murat  United States
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I knew no one likes meee!
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Old 24th February 2005, 06:36 PM   #3
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OK, I'll offer up some opinions and maybe others will chime in

1) - you probably want to keep the buffer if the tone control is going to be driven directly by your CD player. In simplistic terms, the buffer isolates the tone control circuit from the source. If your source does not have a sufficiently low impedance or you have reactive cables you could see some unpredictale interaction with the tone controls.

2) if you try it without the buffer leave in R102, R103 and C100 - connect R102 to the input of the tone stack - the right side of R111. R111 is there to prevent oscillation of the buffer due to the reactive load of the tone stack. R102 will do the same thing for your source. It and R103/C100 form a low pass filter limiting the bandwidth.

3) No you don't have to reinvert the signal. Some people say that you cannot hear absolute phase. My son and I both noticed a difference on a test CD cymbal crash when we switched absolute phase - He asked what the big suckout just before the crash was - and he was just in the room doing something else. I inverted the signal and the suckout sounded like the air rushing out from between the cymbals. You can always just reverse your speaker connections to regain absolute phase rather than include the inverting buffer - Positive speaker terminal to amp's negative, etc.

4) Just leave in the volume pot, C103 and R116 to provide DC blocking. Recognize that if you have long or reactive interconnects you may have some problems.

5) There's a thread in the chip amp forum with listening test results for various op amps (among others). The tester seemed to prefer ADs and said the OPA2132 was better than the 2134. There are those who say that the good old NE5532 sounds just fine. I am happy with the Fairchild version in my active crossovers, but I will be trying others. The high end runs though 7 op amps, the bass 5 in my application. You may want to socket your chip(s) to allow experimentation. It seems to be a matter of personal preference and is very system dependent. Rod designed this circuit for a fet input chip, swapping in a bipolar input chip may have DC offset issues, but that's what the output cap is for.

Hope this helps.
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Old 25th February 2005, 01:59 AM   #4
murat is offline murat  United States
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Of course this helps alot!

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