[sdiy] Tone control questions

Phil Macphail phil.macphail at liivatera.com
Tue Mar 11 11:37:38 CET 2014

Hi David,
What kind of simulations have you run, and what kind of measurements have you made?

I would start with measuring the DC voltages on each of the nodes (the schematic is simple enough to work these out by hand). I wouldn't do anything else until these readings are correct. 
Did you run any transient simulations on the design, or just AC for the frequency response? If it's the latter then is your test signal level low enough to be sure that the amplifiers aren't clipping? This can be hard to spot at the output of a frequency-selective network  as the clipped signal can be filtered by later stages.
If the DC readings are correct and the test signal small enough then I would suspect either wrong wiring or wrong component values.

Good luck

On 11 Mar 2014, at 06:02, "David Ingebretsen" <dingebre at 3dphysics.net> wrote:

> Hello everyone,
> I'm building a Moog 984 4 channel mixer and I'm having problems with the
> tone controls. I found and fixed two errors in the schematic already, the
> resistance of an input attenuator pot was mismarked and one of the resistors
> in the bass control was mismarked as 47k when it should have been 4.7k.
> Anyway, I simulated the circuit in Multisim and with the correct resistor in
> the bass control, it simulates perfectly. When I assembled the PCB, the
> mixer part is fine, but the bass and treble controls don't work right.
> Instead of going from an attenuation at full counterclockwise to neutral at
> mid-point, to boost at full clockwise, I get attenuation at both clockwise
> and counterclockwise and either neutral or boost at midpoint for both
> controls.
> I think I get the general picture of what the tone controls are doing, but I
> don't understand the details. Has anyone any thoughts why the simulation
> would work but not the actual circuit?
> I've done the following:
> 1. Triple checked all the component values on the PCB 2. Quadruple checked
> my schematic against the Moog schematic and the Multisim schematic.
> 3. Replaced the transistors
> 4. Replaced the tone control pots
> 5. Played around a lot with the Multisim simulation trying to get a handle
> on what the various components do and what would happen with a wrong value
> in the tone control circuit.
> 6. Tried to check the dumb things like power supply voltages, etc.
> If anyone wants to see my schematic capture, the moog schematic, or my
> multisim schematic/project I'm happy to send them along.
> Thanks everyone.
> David
> David M. Ingebretsen
> Collision Forensics & Engineering, Inc.
> 2469 E Ft Union Blvd, STE 114
> Salt Lake City, UT 84121
> dingebre at 3dphysics.net
> dingebre at CFandE.com
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