Hello all and Happy New Year!
For any with experience designing Baxandall tone control amps, any tips or observations on this design before I send it off for fab?
Works well in LTSpice and breadboard, but can't hurt to have some feedback while it's still on the drafting board.
Application is a replacement PCB for the tone control amp in my Pioneer SX-750 receiver.
Cheers,
Mike
For any with experience designing Baxandall tone control amps, any tips or observations on this design before I send it off for fab?
Works well in LTSpice and breadboard, but can't hurt to have some feedback while it's still on the drafting board.
Application is a replacement PCB for the tone control amp in my Pioneer SX-750 receiver.
Cheers,
Mike
Your HF cutoff frequency is about 10x too high. Try a 10K treble pot, or 3.3nF coupling from it. And I don't know where you're going to get centre-tapped pots from.
Hi and thanks for checking out my little project.
Unfortunately I have to live with the reality of the "hardware heritage". The volume pot upstream, as well as the tone control pots I'm using (salvaged from other vintage Pioneer receivers) are 100K linear with center tap. So that explains the 100K and center tap choices - not ideal, but they physically work with the metalwork, knobs, etc.
What do you think the roofing filter -3dB point should be? I've gone for about 300kHz. It's not a definite number I've found from Self or Cordell.
I was referring to the treble control cutoff actuallly, which is around 10kHz, should more like 1kHz.
If you haven't cast your design in stone yet, it would be a really good idea to read the manual for the APT Preamp. The PDF is available via search on this site. It contains a very well thought out tone control design and the explanation in the manual is truly a thing of beauty that is not provided in manuals any more. the schematic is provided also.
Thanks. It looks like a well regarded pre-amp design and I'll look forward to going thru the manual and schematic to see what I can glean from it.
From a quick look at the op-amps used, I can see it is fairly old. Luckily now there are better op-amps available.
Cheers, Mike
As said, centretaped pot isn't necessary.
Also I may suggest to change the input stage from noninverting to inverting, as the tone stage is inverting, you preserve input and output phases equal as is usual.
Also C11 and C12 may be ommited because the DC level is the same at both sides.
Also I may suggest to change the input stage from noninverting to inverting, as the tone stage is inverting, you preserve input and output phases equal as is usual.
Also C11 and C12 may be ommited because the DC level is the same at both sides.
I did simulate the Treble pot center-tapped and not, and did not find a difference in results. Since the pots I'm using are center-tapped I'll probably just leave the circuit as designed by PB.
Unfortunately the source impedance from the volume pot upstream of this tone amp board (11k5) gets in the way of using an inverting amplifier as the input amp/buffer. I'm not overly concerned about absolute phase being maintained in this application, though I know it is important in some situations.
Depending on what op-amp I use for the first stage (U3): NE5532, OPA2134 or LM4562, I may end up with some DC offset due to bias current. I don't want that DC flowing into the tone control pots and causing noise when the pots are adjusted. I'll measure the DC offset after choosing op-amps and then decide whether to use blocking caps C11 & C12 or not. Similar for the stabilizing caps for the second op-amp stage (U4) C20&C22 - may /may not need them for HF stability based on the op-amp used there.
l find it easier to put spots for component like those on the PCB ahead of time. Can always leave them unpopulated / jumpered if not needed.
Thanks for looking it over. Mike
Unfortunately the source impedance from the volume pot upstream of this tone amp board (11k5) gets in the way of using an inverting amplifier as the input amp/buffer. I'm not overly concerned about absolute phase being maintained in this application, though I know it is important in some situations.
Depending on what op-amp I use for the first stage (U3): NE5532, OPA2134 or LM4562, I may end up with some DC offset due to bias current. I don't want that DC flowing into the tone control pots and causing noise when the pots are adjusted. I'll measure the DC offset after choosing op-amps and then decide whether to use blocking caps C11 & C12 or not. Similar for the stabilizing caps for the second op-amp stage (U4) C20&C22 - may /may not need them for HF stability based on the op-amp used there.
l find it easier to put spots for component like those on the PCB ahead of time. Can always leave them unpopulated / jumpered if not needed.
Thanks for looking it over. Mike
Yes, the TI opamps are very dated. Just be careful slapping in newer, faster ones. Mooly has some comments on ths board about watching for oscillations when swapping opamps. The old TI parts are very forgiving of non-optimal circumstances involving layout, wiring, supply bypassing, etc.
Good luck!
Good point. I demonstrated that when installing the Ver.1 of this board in the receiver. I hooked it up and touched a 10X scope probe to the board's signal output where it connected to the main amp PCB via a 6" length of shielded signal cable to check for a clean sine wave on the scope. Instead I got an instantaneous little puff of smoke from one of the Zobel resistors on the main amp board. Poof.
On Ver 1 of the tone amp board I had neglected to include the 47ohm output resistors on the final op-amp outputs, figuring that 6" of shielded cable would not present enough capacitance to cause stability problems. Apparently adding the few pF of the scope probe was enough to tip that op-amp into ringing. Lesson learned!
On this Version.2 I've included the 47ohm output resistors and a provision for a stability feedback caps around the feedback op-amps. Just to be sure
Rod Elliot's calculations are incorrect, as his own graph and text show. For the correct calculations see the LME49740 datasheet. Your fHb is about 5kHz, nearly ten times as high as you want it. You've also left out the stopper resistors either side of the treble pot, to flatten out the response at HF.
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yeah, it's a design from the 70s. I wouldn't expect anyone to use TL072 parts these days. however the design principles and topology employed are still valid and very good; worth reviewing for good ideas.
The circuit as given in post #1 shows a bipolar opamp U4 without bias current path through a pot wiper - that's a recipe for loud crackling as the pot is moved as the bias current gets interrupted due to wiper intermittency - the original version I gather had FET opamps TL072 - that wasn't arbitrary.
U4 is intended to be FET input, in order to avoid having to provide a separate DC bias path not through the wiper of RV1. With a new pot this might not make a difference, but the noises when they happen are loud (full scale 15V excursions).
U4 is intended to be FET input, in order to avoid having to provide a separate DC bias path not through the wiper of RV1. With a new pot this might not make a difference, but the noises when they happen are loud (full scale 15V excursions).