Low-distortion Audio-range Oscillator

[1audio]

Yes, if you can make it stable. That's the premise of the LTC super oscillator.

[davada]

Originally Posted by RNMarsh

What would happen using another opamp in series with the oscillator opamp in such a way that it is one super high OLG opamp -- would the thd be even lower? Thx-RNMarsh

Composite amplifiers are possible but are usually composed from two different type amplifiers. Voltage feedback + current feedback for example.

The LT supper oscillator is a tuned composite amplifier to make it work at 10KHz.
Generally the compensation of two amplifiers in series raps the phase around through 180 deg and beyond before reaching unity. So they are unstable. We would like to see a slowing down of the rate of change of phase rather than an speed up. If the slope increases so does the rate of change of phase. Second order compensation networks can be made to work which provide extended bandwidth.

But what makes you think this is the cause of limit?

[RNMarsh]

there are two places to reduce distortion in the oscillator circuit (neglecting passive parts, contacts etc):
The opamp and the jFET. I replaced the jFET with a mulit-turn control trim pot; I could tune the distortion to under .001% THD+N. I ran out of control sensitivity and need a smaller R value with 28 turns to fine tune it further. I used a 100 ohm trim pot. With the jFET out of the picture, THD changes with opamp's can be learned.

Happy Holidays, Everyone !! Take care of your health is the best present you can give. Thx -RNMarsh

[mkc]

Originally Posted by davada

QuantAsylum was talking first release of software in the next few weeks but I don't know about the Hardware.

You can Email support at QuantaSylum and ask.

support@quantasylum.com

Cheers,

Thanks, I have send them a mail.

May I ask why you are buying a E-MU 0204?

Best regards,
Mogens

[mkc]

Originally Posted by 1audio

The box is ideally suited to mobile electronics but needs external stuff for traditional audio. I am working on a solution for my needs.

Hi Demian,

I realize you wrote "for my needs", but is it something you would share in case it's not to specialized for your needs?

I assume it might be some attenuator and filter front end?

Best regards,
Mogens

[davada]

Originally Posted by mkc

Thanks, I have send them a mail.

May I ask why you are buying a E-MU 0204?

Best regards,
Mogens

Hi Mogens,

This is a beta program with the QA400. I need something to compare to.
The EMU0204 and ARTA is something I am familiar with and I know what the base line is.

The QA400 in the end will be the better of the two but it not quite there yet.

In addition to Demian's comment. I believe QuantAsylum is working on a front end solution of their own.
It's been mentioned to them often enough. They have a specific target for this product and that is fine.

For now all anyone really needs to get going is a Twin T or other notch and an attenuator at the input to the notch which can be as simple as a wire wound pot. For smaller signals some amplification might be necessary but not essential. For other needs you might want a balanced input converted to single by shorting one input. Some filtering would be nice but that would only make the FFT look better and define a smaller bandwidth.

I would like galvanic isolation myself.

All these things are difficult to do with out adding to the baseline. Add-on's are probably the way to go. Then we are not stuck with one or two confuguarations.

Cheers

[1audio]

This is a great combination of hardware and software. It does what its intended to do. There are still some bugs in the calculations of levels etc, They are actively working on them.

It has two real limitations. First comes from not acting like an audio interface so other software doesn't work with it. For now all you get is the software they have created. Second, the input/output levels are what I would call mobile electronics levels; 1V max.

I am drafting an expresspcb circuit/layout to address this. First differential inputs, then differential attenuator to handle up to 100V rms and finally an active balanced ground adaptive output with up to 10V single ended/balanced out. Since the QA400 has a common ground for the 4 i/o's, differential connections are essential. Then getting accurate calibrate-able levels throughout is important.

Both of these are more complex than they appear,

It is conceivable that this could be powered from USB but I'm not sure yet. I need to find a good source for 5V to +/- 18 v dc-dc converters that are not real expensive. It may be possible to use a charge pump. However the grounding issues may make that impossible.

I'll post my work in progress later. . .

[mkc]

Hi,

Thank you both for your detailed answers.

It would be great if this thread could come to include a design for a front-end to the QA400 that would make it easy to perform comparative measurements. Perhaps this could even become a diyaudio standard.

I have a 1 and 10 KHz version of Victors oscillator, together with a @juli and a E-MU tracker Pre, which I believe is somewhat similar to the E-MU 0204. But, I haven't got around making any notch and front-end.

I'm sure you know Glen's project: An Audio T.H.D. Analyser

I have been speculating if his measurement front-end and filter could be simplified somewhat and adapted to match the soundcard interface. I this case it could be the QA400. Perhaps it's more complex than it actually needs to be.

Best regards,
Mogens

[jackinnj]

Originally Posted by 1audio

It is conceivable that this could be powered from USB but I'm not sure yet. I need to find a good source for 5V to +/- 18 v dc-dc converters that are not real expensive. It may be possible to use a charge pump. However the grounding issues may make that impossible.

Some folks who built Millet's RMS converter experienced noise issues with USB.

If I had to design a piece of audio instrumentation communicating via USB I'd isolate it (Analog Devices has a development kit for around $50, which you can do for a fraction if you SMT, same with other folks), and use a linear supply on the business end.

[davada]

Originally Posted by jackinnj

Some folks who built Millet's RMS converter experienced noise issues with USB.

If I had to design a piece of audio instrumentation communicating via USB I'd isolate it (Analog Devices has a development kit for around $50, which you can do for a fraction if you SMT, same with other folks), and use a linear supply on the business end.

Hi jakinnj,

Dick Moore bought one of those AD demo boards but couldn't use it because it only does full speed. The EMU0204 and QA400 use high speed.

If we could interface to ethernet and back we'd have the galvanic isolation. or maybe something else
balanced.


Cheers,