Hi all,
I have another amp and this time I'd like to know what components and values of them I would have to change in order to modify (cut/boost) the tone control from original 100Hz (bass) and 10kHz (treble) to, for instance, 80Hz (bass) and 12kHz (treble).
Very important to know how it operates. 🙂
If it's only the capacitors or other component as well.
There is also an Opamp in the path (NJM2068DDC). Don't know if this could be replace for a 5532 or a 2134... 😕
Schematics below.
Many thanks in advance. 🙂
I have another amp and this time I'd like to know what components and values of them I would have to change in order to modify (cut/boost) the tone control from original 100Hz (bass) and 10kHz (treble) to, for instance, 80Hz (bass) and 12kHz (treble).
Very important to know how it operates. 🙂
If it's only the capacitors or other component as well.
There is also an Opamp in the path (NJM2068DDC). Don't know if this could be replace for a 5532 or a 2134... 😕
Schematics below.
Many thanks in advance. 🙂
An externally hosted image should be here but it was not working when we last tested it.
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Thanks! 🙂
I see VR305 (30kΩ, bass control) has the electrolytics C317, C318 (0.1uF/50V) and the C319, C320 (0.47uF/50V) around it.
Similarly VR304 (10kΩ, treble control) has "ceramic" C313, C314 (0.033?) and electrolytics C315, C316 (0.22uF/50V).
Not sure IF all of them have to be changed.
Also "bass" resistors R325, R326 (33kΩ) and "treble" resistors R315, R316 (7.5kΩ) also have to be changed...
An externally hosted image should be here but it was not working when we last tested it.
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Change only the capacitors. The Bass caps will need to be slightly larger, the treble caps, slightly smaller.
Ok! 🙂
Something for the Bass around 0.56uF/50V for both C319, C320 and 0.12uF/50V for both C317, C318, right?
Those values should come close. I'm not sure that you'll hear much difference since the values (and frequencies) are quite close to the original design.
I think I got it. Maybe only changing down to 50Hz and something would be heard...
Ok, It's good to know this! 🙂
Increase capacitances by the same amount of the lowering frequency and viceversa, example, a 1µF cap for 100Hz must be 1.25µF for 80Hz.
It is of no good practice to use electrolytics in frequency dependent networks as their capacitance is variable with temperature and aging, and sensible to DC voltages.
It is of no good practice to use electrolytics in frequency dependent networks as their capacitance is variable with temperature and aging, and sensible to DC voltages.
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I noticed that some of the tone capacitors are electrolytics. This is typical of older consumer grade designs. I have replaced the electrolytics (and ceramic) capacitors in the tone circuit of a couple receivers with slight but noticeable improvement in sound. Plus, the caps are typically 20% tolerance (both electrolytics and ceramics) and the replacements are 10%. I have noticed that parts tolerance in consumer grade tone controls is quite wide, especially the potentiometers. So your tweak might not change things as much as you think, but it should at least provide a marginal reduction of distortion.
If anybody doesn't believe me about the pots, find an old one out of a receiver and measure it across the range. If you find one within 10% across the board that's pretty good. Also notice that it's often impossible to find a true center for both sweeps simultaneously, which is unfortunate.
I honestly don't remember what type of caps I put in (they were from Rat Shack), but surely we can do better than that.
If you want to get fancy, consider providing two turnover frequencies switched by a DPDT switch.
If anybody doesn't believe me about the pots, find an old one out of a receiver and measure it across the range. If you find one within 10% across the board that's pretty good. Also notice that it's often impossible to find a true center for both sweeps simultaneously, which is unfortunate.
I honestly don't remember what type of caps I put in (they were from Rat Shack), but surely we can do better than that.
If you want to get fancy, consider providing two turnover frequencies switched by a DPDT switch.
Not only that, but electrolytics introduce distortion when they have a voltage across them. They are a terrible choice for any frequency dependent network. But as I pointed out in my post, this is typical of a whole lot of older consumer grade equipment.
Certainly, but sometimes they are unavoidable. Case of a cathode decoupling, although in my actual design I use no electrolytics at all: al stage is push pull son cathode decoupling is unncessary and benefical.
Fantastic... 🙂
I use to replace all the low value lytics (from 2uF to lower) to HQ and stable film caps like metallized wound and stacked versions. 🙂
That's good to know. I knew electrolytics are not the primary choice for crossover networks because of high instability and "buzz" noise, but in this case anything helping to reduce distortion is a gain. Do you recommend better potentiometers for the VR305 (30kΩ, bass control) and VR304 (10kΩ, treble control)?
An externally hosted image should be here but it was not working when we last tested it.
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An externally hosted image should be here but it was not working when we last tested it.
Super!
The only thing I know is that they are +- 8dB
Decoupling is completely different than a frequency dependent circuit like a filter. An AC voltage is developed across the cap in a filter. An electrolytic will introduce no appreciable distortion if it is used in a circuit where there will be no AC voltage across it in the audible range.
I don't think there's much to gain in replacing the pots. If you really want precision, you could use precision resistors and a rotary switch. But the real bang for the buck (and effort) is to replace the caps with something better. Save the fancy stuff for when you build a circuit from scratch.
I always reuse old pots out of receivers if they're in OK condition. Their lack of precision doesn't necessarily translate into much in practical terms. You can always add a defeat switch if you must have true flat response.
Thanks for yours advices, folks.
Pictures of the tone control board below...
Above the tone control board and a close-up with the pots and the eight caps (2 film caps and 6 electrolytics).
I'll modify/play the tone control (bass) with another frequencies here.
The Bass part with 0.1uF and 0.47uF lytic caps that will be replace with some Wimas/Rifas.
The Treble part. Here I'm not sure if change frequency band, or only the lytic for some Rifa of the same values.
Finaly the opamp 2068DD. Again, not sure if worth the effort to put a new NE5532 I can take for a few coins.. 😉
Pictures of the tone control board below...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Above the tone control board and a close-up with the pots and the eight caps (2 film caps and 6 electrolytics).
I'll modify/play the tone control (bass) with another frequencies here.
An externally hosted image should be here but it was not working when we last tested it.
The Bass part with 0.1uF and 0.47uF lytic caps that will be replace with some Wimas/Rifas.
An externally hosted image should be here but it was not working when we last tested it.
The Treble part. Here I'm not sure if change frequency band, or only the lytic for some Rifa of the same values.
An externally hosted image should be here but it was not working when we last tested it.
Finaly the opamp 2068DD. Again, not sure if worth the effort to put a new NE5532 I can take for a few coins.. 😉
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