I need help with impedance matching for PC test rig

[CloroxCowboy]

Hi all,

I recently had an idea (I'm sure someone has probably thought of this before...) for how to use my computer as an all in one test rig for my DIY guitar amps. The general idea is this:

1) Record a short sample from my guitar straight into the PC. I want this sound to be completely raw...no effects...no preamp...only the signal that my guitar amp would see if I plugged the guitar straight in to the amp input.

2) Connect the line-out from the PC to the input of the guitar amp being tested, and play the recorded sound sample from step 1.

3) Use a probe to record the signal from different points in the amp circuit (after 1st preamp stage, 2nd stage, each side of the phase inverter, after the output tubes, etc...) and send this signal back to my PC (if it's capable of playing and recording at the same time??) or to a second PC if necessary.

4) Analyze the waveforms of the signals from step 3 and tweak the amp circuit as desired.


The reason I want to do all this from a recorded guitar signal is because I can compare the before and after of any tweak that I perform and see EXACTLY what change it caused by simply subtracting one waveform from the other, and doing Fourier transforms, etc.

I know how to do all that waveform analysis, but what I'm not very sure about is how to match all the impedances from the various inputs and outputs so that this will record and playback "realistically". For example, I know that connecting my guitar straight to the PC microphone input is an impedance mismatch and will not record the same signal that the amp would see. Also, I'm sure I'll need to match the impedances on steps 2 and 3.

So can anyone help me figure out how to build the 3 DI boxes I would need for this setup? I'm ashamed to say that I'm not even sure how to measure the impedances of these inputs and outputs...I understand the technical definition of impedance but that's about it. Tried to find some specs for my sound card impedances but it's a cheapo so the manual doesn't go into that level of detail.

Any help or ideas would be appreciated.

[Enzo]

Welcome to the forum.

Impedances are not the only concern. Do you really have a way to interpret the differences in waveforms in terms of how they SOUND? I mean, if measured distortion is up 25%, is it desirable distortion, or not?

Or said another way: OK, we now see a difference waveform - kinda a little spiky thing with lumps between the spikies. Now what?


I think all you need as an input is a high Z input - something that won;t load down your signal. And for that matter a Hi-Z output should work into an amp input pretty well. Get a sound card with Hi-Z line ins and outs. Don;t try to use a headphones output for anything. And your mic input is also not really suited for guitar.

You could also just record straight guitar into a tape deck or HDD recorder unit, separate from your computer.

[Steve Conner]

If you want to probe nodes inside a tube amp without affecting the signals you're measuring, you need a really high impedance. 1 Meg wouldn't be too much. You also need some sort of attenuator, because signals in an overdriven tube preamp can be in the 100s of volts.

If I were you, I'd use an active DI box with 1 Meg input impedance, and plug in a switchable x1/x10 scope probe using a 1/4" to BNC adapter. The same DI box, minus the probe, will do for recording electric guitar.

When connecting computers to audio gear, you also have to be very careful with ground loops. A computer motherboard can inject a lot of noise into a ground loop.

DIY hi-fi amp builders commonly use a PC running something like Rightmark Audio Analyzer to check their SNR and THD, and it's not uncommon to see ground noise from the setup swamping the noise and distortion they want to measure. Balanced ins and outs help here. Pete Millett designed a dedicated I/O box for this job: Soundcard Interface

I've never seen it done with a guitar amp, apart from some brief experiments I did measuring tone control response with a FFT analyzer. But it should be easier than hi-fi because the distortion is bigger.

When comparing waveforms, be aware that all soundcards have sigma-delta ADCs whose digital filters add a lot of ringing to the edges of square waves. http://www.dspguide.com/ch11/4.htm

As Enzo rightly mentions, the ultimate goal with guitar amps is subjective pleasure. You don't always need to know what the waveforms look like, as long as they sound good and nothing is smoking. Looking at the waveforms is more useful for troubleshooting, or for educational purposes, if you want to learn how the look of a waveform relates to its sound. That is useful to know though, and can help you as an amp designer.

[CloroxCowboy]

Thanks to both of you for the responses. If I understand what you're saying correctly, as long as the load impedance is larger than the source impedance then there shouldn't be any loss of signal? Do I need to try and match them exactly?

As far as waveform analysis, I agree it is completely subjective. I know which amps I prefer and that may be totally different than someone else. But if I can understand the makeup of the sounds I prefer in terms of harmonics, feedback, etc and if I can see EXACTLY how different adjustments affect those sound components then I can modify the amp to sound better to me, but not necessarily anyone else. So I guess you could call it an educational exercise.

Enzo, seeing the differences in a waveform in the time domain is only part of the process. I would also use the Fourier transform to get the signal into the frequency domain which will show me the type of distortion (2nd, 3rd, 4th harmonics, etc). BTW, here's a link if you're interested in what I mean:

Fourier transform - Wikipedia, the free encyclopedia

Thanks again.

[Enzo]

I understand that, and I was not trying to discourage you from it, just wanting to look at things with the goals in mind.

[raiken]

Thanks to both of you for the responses. If I understand what you're saying correctly, as long as the load impedance is larger than the source impedance then there shouldn't be any loss of signal? Do I need to try and match them exactly?

If you match the impedance exactly, you will lose exactly half your signal voltage. You don't want to do that. A general "rule of thumb" for bridging load is that the impedance needs to be at least 10 times the source impedance to be considered negligible. At this load, you will lose around 10% of the signal voltage. Your typical scope probe is 10Meg input impedance.

In small-signal voltage amplifiers, you don't normally match impedances, you strive for the ideal, which is a zero output impedance and an infinite input impedance. This allows for maximum voltage transfer.

Enzo, seeing the differences in a waveform in the time domain is only part of the process. I would also use the Fourier transform to get the signal into the frequency domain which will show me the type of distortion (2nd, 3rd, 4th harmonics, etc).

The easiest way to do this is to get a scope with a vertical output amplifier BNC on the back, such as most older Tek scopes like the 2445/2465, etc. have. Then you just probe the amp node with your scope probe, adjust the vertical gain section of the scope for proper level, and record the output of the vertical output BNC signal.

If you don't have such a scope, I'd recommend building a small buffer box that has a high-voltage capacitor AC-coupling the input into a high-impedance (10Meg) 10:1 attenuator to protect things and cut the signal down to a reasonable level and not load your source. Then run that into a pot or stepped attenuator to adjust the levels and into an opamp buffer to provide a low-impedance drive to your PC input.

Randall Aiken

[CloroxCowboy]

If you don't have such a scope, I'd recommend building a small buffer box that has a high-voltage capacitor AC-coupling the input into a high-impedance (10Meg) 10:1 attenuator to protect things and cut the signal down to a reasonable level and not load your source. Then run that into a pot or stepped attenuator to adjust the levels and into an opamp buffer to provide a low-impedance drive to your PC input.

Randall Aiken

Thanks Randall. Do you know of an example schematic online for a simple opamp buffer? I've never worked much with those components...just tube amp stuff so I'd want to make sure I do that part correctly.

[raiken]

Thanks Randall. Do you know of an example schematic online for a simple opamp buffer? I've never worked much with those components...just tube amp stuff so I'd want to make sure I do that part correctly.

Do a search for "opamp buffer" and you'll find a zillion of them. They can be either single-supply or dual-supply voltage circuits, pick what suits you best.

RA