Hi, diy community people,
I have an old crown preamp based on 70's op-amps and looking at its schematic, noticed one thing strange.
schematic
It uses one single channel op-amp for each channel in non-inverted configuration, and it has tone control feature. As far as my limited experience tells me, I've never seen tone control circuit in this configuration, which amazes me a bit. All circuits I saw had inverted tone control then another inverted unity gain for phase correction..
Perhaps the engineer of this unit wanted to minimize the number of op-amps by non-inverted tone control followed by no 2nd stage. Is it considered a good technique, I wonder?
I'm also feeling this preamp sounds very dry. Basically it has no gain throughout the circuit which makes it sound dull, I suppose? There's a switch I can select 0db and -10db. At 0db, I could hear noticeable noise. Replacing with OPA627 didn't help.
Any suggestion for possible mod and opinions on the circuit design will be truly grateful. Thanks,
I have an old crown preamp based on 70's op-amps and looking at its schematic, noticed one thing strange.
schematic
It uses one single channel op-amp for each channel in non-inverted configuration, and it has tone control feature. As far as my limited experience tells me, I've never seen tone control circuit in this configuration, which amazes me a bit. All circuits I saw had inverted tone control then another inverted unity gain for phase correction..
Perhaps the engineer of this unit wanted to minimize the number of op-amps by non-inverted tone control followed by no 2nd stage. Is it considered a good technique, I wonder?
I'm also feeling this preamp sounds very dry. Basically it has no gain throughout the circuit which makes it sound dull, I suppose? There's a switch I can select 0db and -10db. At 0db, I could hear noticeable noise. Replacing with OPA627 didn't help.
Any suggestion for possible mod and opinions on the circuit design will be truly grateful. Thanks,
I have looked for tone control circuits with opamps in web, but mostly found circuits with tone control opamp in inverting stage, and use another inverting opamp buffer before or after it, to get the signal to original phase.
eg. http://sound.westhost.com/project97.htm
For the preamp , I am looking for a buffer opamp stage (SSP) followed by a switch which will be used to select to connect to output or to tone control opamp stage. So making the tone control optional, and only use it in signal path , when it is needed.
But when we are switching on to use the tone control stage, as here the tone control opamp is inverting stage, then we need another output inverting buffer to make the single in phase again. So, while using tone controls can we avoid the output buffer stage which is needed only to invert the signal because of the inverting tone control stage? Can we have tone controls opamp in non-inverting stage?
Else the first buffer opamp (SSP) needs to be in non-inverting stage, which will compensate for inverting tone control stage, but when we are switching off the tone control stage the output from the first buffer stage (SSP) will be inverting again.
thanks and regards
S Sarath
eg. http://sound.westhost.com/project97.htm
For the preamp , I am looking for a buffer opamp stage (SSP) followed by a switch which will be used to select to connect to output or to tone control opamp stage. So making the tone control optional, and only use it in signal path , when it is needed.
But when we are switching on to use the tone control stage, as here the tone control opamp is inverting stage, then we need another output inverting buffer to make the single in phase again. So, while using tone controls can we avoid the output buffer stage which is needed only to invert the signal because of the inverting tone control stage? Can we have tone controls opamp in non-inverting stage?
Else the first buffer opamp (SSP) needs to be in non-inverting stage, which will compensate for inverting tone control stage, but when we are switching off the tone control stage the output from the first buffer stage (SSP) will be inverting again.
thanks and regards
S Sarath
Active tone controls need to be inverting. If they were not then the tone control feedback would appear as positive feedback and the stage would oscillate.
Passive tone stages exist and are of course non inverting, but they have high attenuation and more gentle boost and cut.
Passive tone stages exist and are of course non inverting, but they have high attenuation and more gentle boost and cut.
Hi Mooly,
Thanks for the clarification.
out of the below 2 options which one would be better:
(here using a switch after input buffer stage to connect to output or tone control stage
if we donot need tone control switch will be set to make the input buffer output as preamp output
if we need tone control switch will be set to connect input buffer output to tone control stage)
1. opamp buffer (zero or little gain) ==> switch ==> passive tone control ==> opamp buffer with little gain
2. opamp buffer (zero or little gain) ==> switch ==> inverting opamp tone control ==> inverting opamp buffer
In the first case only 2 opamp stages will be used where as in 2nd case 3 opamp stages needs to be used...
thanks and regards
S Sarath
Thanks for the clarification.
out of the below 2 options which one would be better:
(here using a switch after input buffer stage to connect to output or tone control stage
if we donot need tone control switch will be set to make the input buffer output as preamp output
if we need tone control switch will be set to connect input buffer output to tone control stage)
1. opamp buffer (zero or little gain) ==> switch ==> passive tone control ==> opamp buffer with little gain
2. opamp buffer (zero or little gain) ==> switch ==> inverting opamp tone control ==> inverting opamp buffer
In the first case only 2 opamp stages will be used where as in 2nd case 3 opamp stages needs to be used...
thanks and regards
S Sarath
The `normal' inverting tone control has unity gain and the same amount of boost and cut, but must be driven from a low source impedance (so an inverting buffer is used here).
However it is possible to feed the input signal into the opamps `+' input and ground the feedback circuit - see attached drawing. The result is a tone control that has a gain of 2 (useful if it is following a volume control), but twice the amount of boost as cut (ie -4dB cut and +8dB of boost in my example). Some simple switches can convert the tone control into a flat gain of 2 stage.
The Crown IC150 preamplifier mentioned in post #1 used a non inverting tone control which had a gain of `11' but used special non linear pots to give similar levels of boost and cut.
Regards,
Paul Bysouth
However it is possible to feed the input signal into the opamps `+' input and ground the feedback circuit - see attached drawing. The result is a tone control that has a gain of 2 (useful if it is following a volume control), but twice the amount of boost as cut (ie -4dB cut and +8dB of boost in my example). Some simple switches can convert the tone control into a flat gain of 2 stage.
The Crown IC150 preamplifier mentioned in post #1 used a non inverting tone control which had a gain of `11' but used special non linear pots to give similar levels of boost and cut.
Regards,
Paul Bysouth
Attachments
Actually you can put the exact same configuration of the tone stack into a non-inverting config, this just increases the gain (hence mid-band gain will be always be significantly greater than 1). With a little tweaking of the values, you can make a perfect tone control with +-6dB or more.
A long time ago someone here posted a +-20dB non-inverting amplifying tone control with the additional feature of a full disconnect of the tone stack with a switch (true bypass), EDIT: but not the one Paul posted.
The only point that is really different is that you can't have unity gain in non-inverting, either it's less (buffered passive) or greater than unity.
EDIT : I recognize Paul just said the same 😉
With a low boost/cut range of +-6dB or less you can get away with linear pots and some added resistors to center the curve at mid position of the wiper.
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Thanks for the clarifications and examples guys, there are usually ways and means 🙂
Personally I would go for a standard configuration If the phase reversal bothers you then adding a second inverting stage will have no bearing on the audio quality.
Personally I would go for a standard configuration If the phase reversal bothers you then adding a second inverting stage will have no bearing on the audio quality.
In the real world,you can get away with two inverting stages preamp and it its going to be in same case as power amps, to have the volume control between preamp and power amp.
Inverting amp buffer ==> Inverting tone control ==> Volume ==> Power amp.
your inverting buffer can, if required, be set for a small amount of gain. 2-5x is the norm, and the resistor values chosen to suit this and your desired input impedance.
For example, 25K ohm input and gain of 4, Ri = 25K; Rf = 100K for first stage, which will double as buffer for the tone control. Remember inverting tone control does have varying input impedance depending on how controls are set, so a buffer before it is practically mandatory!
Under these circumstances, the volume control can be quite low in value, 10K or less. I do not recommend going too high with resistor values as its all too easy to get noisy results.
However it is maybe surprising to note that opamps with their low output impedances still work very well at lower values of load and feedback resistance. Simply put, a normal opamp idling at 2mA in it's output stage will just stop operating in class A with a load as low as about 3K ohm at 2V nominal out....
Inverting amp buffer ==> Inverting tone control ==> Volume ==> Power amp.
your inverting buffer can, if required, be set for a small amount of gain. 2-5x is the norm, and the resistor values chosen to suit this and your desired input impedance.
For example, 25K ohm input and gain of 4, Ri = 25K; Rf = 100K for first stage, which will double as buffer for the tone control. Remember inverting tone control does have varying input impedance depending on how controls are set, so a buffer before it is practically mandatory!
Under these circumstances, the volume control can be quite low in value, 10K or less. I do not recommend going too high with resistor values as its all too easy to get noisy results.
However it is maybe surprising to note that opamps with their low output impedances still work very well at lower values of load and feedback resistance. Simply put, a normal opamp idling at 2mA in it's output stage will just stop operating in class A with a load as low as about 3K ohm at 2V nominal out....
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Maybe it's me - but I cannot see how a tone control can be non-inverting as in the above examples. At very least you will lose the ability to 'cut'.
where xi and xf are the frequency dependant impedances forming "Ri" (input resistor) and "Rf" (feedback resistor) of normal negative feedback op-amp configurations:
Giving example of tone set to 20 dB cut xi=10K, xf = 1K
gain = 1K/10K = 0.1 - fair and good you have 1/10th of the signal output as expected....
same applied to non-inverting amplifier
gain = 1+(1K/10K) = 1.1 - so no cut possible in this configuration!
maybe original circuit was misinterpreted and was in fact passive tone control followed by non inverting buffer - this would make more sense....
where xi and xf are the frequency dependant impedances forming "Ri" (input resistor) and "Rf" (feedback resistor) of normal negative feedback op-amp configurations:
Giving example of tone set to 20 dB cut xi=10K, xf = 1K
gain = 1K/10K = 0.1 - fair and good you have 1/10th of the signal output as expected....
same applied to non-inverting amplifier
gain = 1+(1K/10K) = 1.1 - so no cut possible in this configuration!
maybe original circuit was misinterpreted and was in fact passive tone control followed by non inverting buffer - this would make more sense....
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Is the "Volume" just a "potentiometer" or "potentiometer with opamp buffer"?
Also do we need to have volume control after tone control or can we have something like:
input => potentiometer (volume control) => inverting buffer => inverting tone control buffer => power amp
Or do we need to have volume control after tone control only like below?
input => inverting buffer => inverting tone control buffer => buffer with volume control => power amp
thanks and regards
S Sarath
If you were going to build a seperate preamp with pot output going to "outside world connection" RCA out, then, yes, a non inverting buffer after the volume "pot" is recommended!
In effect it does not matter too much where the volume pot goes.
However with the volume control ("pot" on its' own) furthest along the signal chain, towards output, because it's most normally used as attenuator, helps attenuate noise build up from the previous stages of amplification.
This especially including potentially higher gain stages like tone controls.
Such order typically gives a better signal to noise ratio.
I therefore tend to place it towards last stages or at very least after last gain stage before power amp.
A buffer just after the volume pot would not hurt, but is normally only necessary if you are also taking this point to outside world, as PREAMP outputs, as above.
In effect it does not matter too much where the volume pot goes.
However with the volume control ("pot" on its' own) furthest along the signal chain, towards output, because it's most normally used as attenuator, helps attenuate noise build up from the previous stages of amplification.
This especially including potentially higher gain stages like tone controls.
Such order typically gives a better signal to noise ratio.
I therefore tend to place it towards last stages or at very least after last gain stage before power amp.
A buffer just after the volume pot would not hurt, but is normally only necessary if you are also taking this point to outside world, as PREAMP outputs, as above.
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That's the coolest non-inverting tone control I've seen.
It should be obvious that an inverting tone control is easier to figure out, and the circuit is more symmetrical. Of course it comes with well known caveats.
A typical "bass boost" switch in cheap consumer products is typically just an extra resistor and capacitor inserted in the feedback network with a switch. In fact you can do a lot of basic "tone shaping" with simple networks like this, which should illustrate that a non-inverting tone control is indeed possible.
.... aha! I see - the stage will have the required "flat" amount of gain - with a +/-20db control, the "flat" gain will still be 20dB..... a bit much in some preamp applications.....
+/- 12dB gives approx. 4x which is a bit more more normal, ballpark, at least in hi-fi....
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If anyone is interested, I'm really pleased with this one...http://www.diyaudio.com/forums/chip-amps/199130-usable-tone-control.html
The later versions (a few pages in) have improvements that really make it better than the usual Baxandal types in my humble opinion.
Warning: Some unabashed self promotion may be detected here. 😉
Mike
The later versions (a few pages in) have improvements that really make it better than the usual Baxandal types in my humble opinion.
Warning: Some unabashed self promotion may be detected here. 😉
Mike
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