I have an early 1990's Sherwood receiver model RX-4010R. I currently have the tone board (preamp) removed and am wondering if it is possible to eliminate the circuitry related to bass and treble controls. This is a lower-end receiver, but it does have RCA pre-out and pwr-in jacks. So I could use it as a preamp to drive other amplifiers.
Ideally, I would like to have the simplest circuit possible, eliminating as many unnecessary components a possible.
In the schematic jpg posted below, please disregard the pink and yellow, highlighted electrolytic capacitors and the red "preamp out" marking. Those were done a while back and have nothing to do with the questions I am asking in this post.
But the areas circled green, red, and yellow, do relate to the topic. As can be seen, this design uses two dual opamps. I am not sure which half of the opamp actually drives output, or what determines gain as the resistor values and connections are very different for each half of the opamp. Beyond my understanding, and that is why I came here to ask 🤓
1) is the circuitry circled in green the basis of a simple, opamp based preamp? What more would be required to make this workable?
2) can I eliminate all of the components circled in red?
3) regarding the components circled in yellow, basically whatever is connected to pins 5, 6, and 7 of the opamp, if the tone circuitry is eliminated, does anything need to be connected to those pins? Would there be any harm in using only 1/2 of the opamp?
Ideally, I would like to have the simplest circuit possible, eliminating as many unnecessary components a possible.
In the schematic jpg posted below, please disregard the pink and yellow, highlighted electrolytic capacitors and the red "preamp out" marking. Those were done a while back and have nothing to do with the questions I am asking in this post.
But the areas circled green, red, and yellow, do relate to the topic. As can be seen, this design uses two dual opamps. I am not sure which half of the opamp actually drives output, or what determines gain as the resistor values and connections are very different for each half of the opamp. Beyond my understanding, and that is why I came here to ask 🤓
1) is the circuitry circled in green the basis of a simple, opamp based preamp? What more would be required to make this workable?
2) can I eliminate all of the components circled in red?
3) regarding the components circled in yellow, basically whatever is connected to pins 5, 6, and 7 of the opamp, if the tone circuitry is eliminated, does anything need to be connected to those pins? Would there be any harm in using only 1/2 of the opamp?
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Thank you wg_ski
After pulling C210, should that vacancy be jumpered? I am guessing no.
As per your suggestion, C213 would be driving output, yes?
So then R208 (560 ohm) and R207 (82 ohm) would set gain? Is that correct?
Would gain be changed versus the original configuration? (I don't understand whether pins 5,6,7 have any role currently in gain setting)
Also, per your suggestion, I believe that the unused 1/2 of the opamp (pins 5, 6, and 7) would have a ground reference, but no signal. Is that correct?
After pulling C210, should that vacancy be jumpered? I am guessing no.
As per your suggestion, C213 would be driving output, yes?
So then R208 (560 ohm) and R207 (82 ohm) would set gain? Is that correct?
Would gain be changed versus the original configuration? (I don't understand whether pins 5,6,7 have any role currently in gain setting)
Also, per your suggestion, I believe that the unused 1/2 of the opamp (pins 5, 6, and 7) would have a ground reference, but no signal. Is that correct?
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What about the point where the negative side of C213 connects to the tone circuitry, should that connection be opened?
The 4559 has slew rate of 2 to 4 from different makes...so a faster type may work.
I think you can make a new board, with trim pots for input and output level adjustment, with a new op amp, and use it in place of the existing board.
About 12 is the limit for audio, so stick to 5523 or TL072, 2134 at 20 V/us may cause oscillations in your amp, the main stage may not be designed to accept such a high slew rate (I did not check the circuit).
And wait for the more experienced members to comment, take their advice as well.
Most sources are now digital, and some apps / programs allow digital tone and level control, and the line output level can feed an amplifier.
So a tone control board is a bit superfluous, in my opinion. No need.
I think you can make a new board, with trim pots for input and output level adjustment, with a new op amp, and use it in place of the existing board.
About 12 is the limit for audio, so stick to 5523 or TL072, 2134 at 20 V/us may cause oscillations in your amp, the main stage may not be designed to accept such a high slew rate (I did not check the circuit).
And wait for the more experienced members to comment, take their advice as well.
Most sources are now digital, and some apps / programs allow digital tone and level control, and the line output level can feed an amplifier.
So a tone control board is a bit superfluous, in my opinion. No need.
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@wg_ski Two final points of clarification if you would please:
Should the additional connection point marked in the pic below, between the negative side of the original C210 and the tone circuitry, also be opened?
Should the unused 1/2 of the opamp should have a connection to ground (via pins 5 and 6) but have no connection to signal on any pin?
Thanks again wg_ski and @tomchr
Should the additional connection point marked in the pic below, between the negative side of the original C210 and the tone circuitry, also be opened?
Should the unused 1/2 of the opamp should have a connection to ground (via pins 5 and 6) but have no connection to signal on any pin?
Thanks again wg_ski and @tomchr
@tomchr thanks for that clarification and for taking the time to edit my pic
It has been pointed out by @wg_ski that the pin 5,6,7 half of the opamp is at unity gain when tone pots are centered. Based on that, I assume that all gain in this preamp is produced by the pin 1,2,3 portion of the opamp.
Guys, another question as I notice that in original configuration, the signal output from C213 is routed through several large value resistors. Eliminating the tone block from the circuit will also remove R212, R210, and R213, in addition to the pots themselves. Do those resistors attenuate signal level?
Wouldn't removing those resistors from the signal path have the effect of increasing output gain? Not sure if "gain" is the proper term, but if this modification is done as described, would the volume knob of the receiver need to be turned CCW to match volume vs the original configuration?
It has been pointed out by @wg_ski that the pin 5,6,7 half of the opamp is at unity gain when tone pots are centered. Based on that, I assume that all gain in this preamp is produced by the pin 1,2,3 portion of the opamp.
Guys, another question as I notice that in original configuration, the signal output from C213 is routed through several large value resistors. Eliminating the tone block from the circuit will also remove R212, R210, and R213, in addition to the pots themselves. Do those resistors attenuate signal level?
Wouldn't removing those resistors from the signal path have the effect of increasing output gain? Not sure if "gain" is the proper term, but if this modification is done as described, would the volume knob of the receiver need to be turned CCW to match volume vs the original configuration?
Hi Tom. Yes, I understand that will be the case if the suggested mods are implemented.
But in original configuration, signal went through several large value resistors. It was not straight wire from pin 1 of IC201 to the output. I am asking if removing those resistors from the signal path would cause signal output to be greater than it had been in the original configuration. I am wondering about any possible changes that might result from changing the circuit.
But in original configuration, signal went through several large value resistors. It was not straight wire from pin 1 of IC201 to the output. I am asking if removing those resistors from the signal path would cause signal output to be greater than it had been in the original configuration. I am wondering about any possible changes that might result from changing the circuit.
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Speaking of gain, that 2.2M in the feedback of the output op-amps seems suspiciously high. Perhaps you'll only loose the noise from that last op-amp being run like that making the changes described.
@tomchr
In my post above I am asking about the resistors marked in green below. I believe that those are in the signal path in the original circuit configuration, but would be replaced with straight wire by implementing suggesting modifications. Is that the case? If so, wouldn't removing those resistors from the signal path alter the output signal level?
In other words, would overall gain structure be affected by implementing the modifications?
In my post above I am asking about the resistors marked in green below. I believe that those are in the signal path in the original circuit configuration, but would be replaced with straight wire by implementing suggesting modifications. Is that the case? If so, wouldn't removing those resistors from the signal path alter the output signal level?
In other words, would overall gain structure be affected by implementing the modifications?
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That was just to provide a feedback path when/if the main feedback path through the pots gets noisy/intermittent. The pot should dominate in normal operation.
He basically wants to remove the circuit and use it elsewhere...would a new board be a better choice instead?
With new op amps and so on?
With new op amps and so on?
@NareshBrd
No, I don't want to use the circuit elsewhere. I want to improve the original preamp circuit of an older receiver that I am currently recapping. I never use tone controls and, no doubt, the pots used are cheap and as far as I can tell, always in the signal path. Getting rid of the pots, and any unnecessary components, is what I am exploring.
This thread is for learning about (this particular) circuit design and modification. Not new opamps. Creating a new board is not even on my radar.
But thank you for asking about that 2.2M resistor. I was curious about why that value was chosen.
No, I don't want to use the circuit elsewhere. I want to improve the original preamp circuit of an older receiver that I am currently recapping. I never use tone controls and, no doubt, the pots used are cheap and as far as I can tell, always in the signal path. Getting rid of the pots, and any unnecessary components, is what I am exploring.
This thread is for learning about (this particular) circuit design and modification. Not new opamps. Creating a new board is not even on my radar.
But thank you for asking about that 2.2M resistor. I was curious about why that value was chosen.
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Right. And those along with the latter half of IC201 ensured unity gain when the tone control pots were centred. Of course, you'll get either boost or cut when you take the pots off centre because that's how tone controls work. 😉
Tom
I'm curious about that as well. But the 220 kΩ across the opamp inputs are even more intriguing. Then again, the impedances in the tone control circuit are pretty high. That probably drove those choices.
Tom