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ESP P97
ESP P97
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Old 15th April 2019, 07:33 AM   #1
nirupambhowmick is offline nirupambhowmick  India
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Question ESP P97

Hi...I have a question, & I think diyaudio forum can help me. Don't you think at the end of the tone control stage a capacitor is needed? I mean before volume control?
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Old 15th April 2019, 08:42 AM   #2
mchambin is offline mchambin  France
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P97 The input stage is configured as shown with a gain of 2 (6dB)

Little DC at the input, is x2 at the output.

Following circuit that does balance has coupling caps.

Conclusion: No need for more caps.
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Old 15th April 2019, 04:08 PM   #3
nirupambhowmick is offline nirupambhowmick  India
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alright, thnks.
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Old 15th April 2019, 04:17 PM   #4
nirupambhowmick is offline nirupambhowmick  India
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the volume potentiometer is before the output stage, this is why i had doubt. i don't like the screechy noise of the potentiometer.
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Old 16th April 2019, 09:00 AM   #5
sgrossklass is offline sgrossklass  Germany
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I wouldn't be worried about the tone stage. Its DC offset is going to be maybe 50 mV, and in general this is going to be attenuated significantly as a typical volume pot setting is around -40 dB.

The stage I'd actually be worried about is the volume control, as the NE5532 has fairly large input bias current. I would suggest adding 6.8 caps in series with R112/212. In return you can pretty much leave out C103/203, these are not nearly as critical.
Alternatively, use an opamp with bias current cancellation (LM4562 etc.) or a low-noise FET input part (e.g. OPA2132/34) in this position. Even a classic bipolar with low input bias current (NJM4565, 4560, even a 4558) should work alright - it's not like you're asking for huge amounts of output level, gain or output current here. Even a classic JFET input part may be adequate (e.g. TL072, AD712, TLE2072), but depending on power amp gain and speaker sensitivity might end up a bit on the noisy side.
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Old 16th April 2019, 06:23 PM   #6
PRR is offline PRR  United States
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> i don't like the screechy noise of the potentiometer.

Please measure and report the DC voltage on all terminals of the potentiometer.
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Old 17th April 2019, 07:48 AM   #7
nirupambhowmick is offline nirupambhowmick  India
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Quote:
Originally Posted by sgrossklass View Post
I wouldn't be worried about the tone stage. Its DC offset is going to be maybe 50 mV, and in general this is going to be attenuated significantly as a typical volume pot setting is around -40 dB.

The stage I'd actually be worried about is the volume control, as the NE5532 has fairly large input bias current. I would suggest adding 6.8 caps in series with R112/212. In return you can pretty much leave out C103/203, these are not nearly as critical.
Alternatively, use an opamp with bias current cancellation (LM4562 etc.) or a low-noise FET input part (e.g. OPA2132/34) in this position. Even a classic bipolar with low input bias current (NJM4565, 4560, even a 4558) should work alright - it's not like you're asking for huge amounts of output level, gain or output current here. Even a classic JFET input part may be adequate (e.g. TL072, AD712, TLE2072), but depending on power amp gain and speaker sensitivity might end up a bit on the noisy side.
In project88 (preamp) Rod says; "if you use ne5532 you may find that the small dc offset voltage at the outputs causes some potentiometer noise. if this occurs, use 10uf electrolytic cap in series with the connections to the balance & volume pot(to the next op-amp stage)."
But in P97 he didn't mention anything about these.
this is the reason of my thread.
Btw thanks for the inputs. I have some genuine ne5532, but i think i should go for tl072.
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Old 17th April 2019, 07:51 AM   #8
nirupambhowmick is offline nirupambhowmick  India
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Quote:
Originally Posted by PRR View Post
> i don't like the screechy noise of the potentiometer.

Please measure and report the DC voltage on all terminals of the potentiometer.
Actually i am going to make this pre :-)
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Old 17th April 2019, 11:51 AM   #9
nirupambhowmick is offline nirupambhowmick  India
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if you don't mind i wish to know another thing.
To block dc offset & to eliminate input bias current problem can i add 2uf film cap before volume & balance control without effecting the performance?
actually i don't want to lose 5532, as you know 072 is inferior to 5532 in comparison of noise characteristic, & it's very hard to get good quality 6.8uf cap here. The size of the 6.8uf capacitor would be another problem.
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Old 18th April 2019, 10:03 AM   #10
nirupambhowmick is offline nirupambhowmick  India
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Hello... i don't know much about electronics, i need your help. Douglas self in his book 'Small signal audio design' says- "It is never a good idea to have dc flowing through any part of a potentiometer. Feeding a bias current through a wiper to the next stage tends to create more serious noise.
This practice is often acceptable for FET op-amps like Tl072, but it is definitely not a good idea for bipolar op-amps such as the 5532 because bias current is much greater. Therefore AC coupling is essential"
i don't have 6.8uf; but have some good quality 1uf caps & i want to use them in parallel. Can i use them in both side of the volume potentiometer as Douglas self suggest?
i dont know about coupling time constant thing. Do you thing 2uf would be adequate? Help me.
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