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Old 28th September 2012, 09:16 PM   #541
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What about this one? It's a version of a bridge T oscillator I came up with that has an rms->dc converter (AD637) with a VTL5C10 as the variable element in the positive feedback leg of U1. U2 is an error amplifier and the variable Vref sets the output level. It's just an outline, obviously. Please feel free to rip it to pieces...

I just realized that the AD637 has a built in buffer, so BUF isn't needed.
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File Type: jpg Bridge T oscillator 001.jpg (46.2 KB, 225 views)

Last edited by dirkwright; 28th September 2012 at 09:24 PM.
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Old 28th September 2012, 09:20 PM   #542
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Quote:
Originally Posted by Bob Cordell View Post
Hi Dirkwright,

Thank you for your very kind words. Look for some new info on the website very soon. As you may know, I published an article in Linear Audio this month describing my VinylTrak MM/MC phono preamplifier. There is a bunch of supplemental information on the design that will go up on my website shortly.

In the meantime, if you love audio I very strongly suggest you subscribe to Jan Didden's wonderful Linear Audio publication, out every 6 months. It just hit the 200-page mark. Check it out at www.linearaudio.net - home. Jan and I will both be at RMAF in Denver, a great audio show. Jan will have an exhibit table there.

Cheers,
Bob
You're welcome. You've been in audio far longer than me.

I have purchased a couple of Jan's output and I agree they are wonderful.
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Old 28th September 2012, 11:05 PM   #543
raul_77 is offline raul_77  Europe
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This are the page of the design show in this post:

Low-distortion Audio-range Oscillator

Low THD Audio Oscillator
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Old 29th September 2012, 01:06 AM   #544
davada is offline davada  Canada
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Quote:
Originally Posted by Bob Cordell View Post
Hi DB,

This is a nice circuit, and may prove very worthwhile.

The full-wave detector in my oscillator is not a true peak detector because it has a significant 10k resistor between the FW rectifier output (from two open-emitters) and the agc filtering capacitors. This seems to work better than a true peak detector in this position.

Although I always worried a lot about the agc ripple, it does not appear to be the limiting issue in distortion in my oscillator, but rather the small distortion of the JFET in the agc. There is a noise/distortion tradeoff there. The fact that agc is not a big problem for me is partly due to the fact that that the agc filter capacitance is range-selected and that anti-parallel diodes greatly reduce the agc feedback amplitude once the oscillator has settled.

Another approach is available for reduced agc jitter, and this is a result of the beauty of the SV providing in-phase and quadrature outputs. Instead of just using a full-wave rectifier (two phases), one can use a 4-phase detector by connecting FWRs to the quadrature output as well (as long as level differences between the quadrature and in-phase outputs are close to being the same). If one wanted to go to a real extreme, they could go to 8 phase detection by combining quadrature and in-phase outputs to form yet another set of detector signals.

Cheers,
Bob
Hi Bob,

Thanks for your comment.

I wasn't singling out your oscillator as benefiting from this TH. I was just using it as an example of a very popular and high performance SVO.

I tried the quadrature peak detector approach a few years back and found that it requires
a lot of calibration. If the balance of the signal is out even a small amount from gain differences, offsets and mismatched diode the output can look very ugly. Although fluke proved that this can be a very reliable approach. I can't imagine trying it with eight.

If one wanted to get crazy with this TH it could be done with the quadrature arrangement. The quadrature stuff can get pretty noisy and requires a lot of filtering.

I'm looking for something that's simple, quiet and non intrusive to the oscillator. But I think I will be hard pressed to come up with something that outperform
your design.

I posted this circuit to see what might come out of it.

Cheers,
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Last edited by davada; 29th September 2012 at 01:13 AM.
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Old 29th September 2012, 06:06 AM   #545
RNMarsh is offline RNMarsh  United States
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Is there any way to get any of these osc and thd meters that are built and not have to build the whole thing myself? I need sources and thd that measure well below .001% .0001 would do it for me. I have no time to make chassis and order parts and wire it and all that. is my only choise to buy an HP or A-P or the like for a lot of $$$ ??
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Old 29th September 2012, 07:06 AM   #546
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Is there any way to get any of these osc and thd meters that are built and not have to build the whole thing myself? I need sources and thd that measure well below .001% .0001 would do it for me. I have no time to make chassis and order parts and wire it and all that. is my only choise to buy an HP or A-P or the like for a lot of $$$ ??
Hi RNMarsh,

It seems we're all in the same boat for time.
Maybe someone here would be willing to build you something.
Or just house one of Victor's.
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Old 29th September 2012, 01:06 PM   #547
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Quote:
Originally Posted by RNMarsh View Post
Is there any way to get any of these osc and thd meters that are built and not have to build the whole thing myself? I need sources and thd that measure well below .001% .0001 would do it for me. I have no time to make chassis and order parts and wire it and all that. is my only choise to buy an HP or A-P or the like for a lot of $$$ ??
You're right about the $$ for decent test equipment. Not being able to afford it is why I began building the stuff myself. However, often there is a lot of good stuff on ebay - although 0.0001% THD stuff is not even widely available in commercial stuff.

Cheers,
Bob
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Old 29th September 2012, 01:26 PM   #548
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Quote:
Originally Posted by RNMarsh View Post
Is there any way to get any of these osc and thd meters that are built and not have to build the whole thing myself? I need sources and thd that measure well below .001% .0001 would do it for me. I have no time to make chassis and order parts and wire it and all that. is my only choise to buy an HP or A-P or the like for a lot of $$$ ??
There are a few HP 339A's on eBay. Everything from a busted one for $120 to a bench queen for $700.

Otherwise, you could hire one of the people on this message board to build something for you.

Last edited by dirkwright; 29th September 2012 at 01:35 PM.
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Old 29th September 2012, 01:58 PM   #549
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Originally Posted by davada View Post
Hi RNMarsh,

It seems we're all in the same boat for time.
Maybe someone here would be willing to build you something.
Or just house one of Victor's.
Speaking of Victor's oscillator, I was wondering if it could be modified to run on +/-5V instead of +/-15V. If it's ok to change the required resistors in the regulator to run on +/- 5V then I could use a couple of 9V batteries and put it in a compact box. Obviously, the output voltage would be much less but that's probably ok. As it is, it's really hard to get a compact power supply for it and 35V is a weird value that doesn't work for batteries. I got his 10kHz oscillator yesterday but I don't know what I'm going to do with it yet.
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Old 29th September 2012, 02:48 PM   #550
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Quote:
Originally Posted by dirkwright View Post
Speaking of Victor's oscillator, I was wondering if it could be modified to run on +/-5V instead of +/-15V. If it's ok to change the required resistors in the regulator to run on +/- 5V then I could use a couple of 9V batteries and put it in a compact box. Obviously, the output voltage would be much less but that's probably ok. As it is, it's really hard to get a compact power supply for it and 35V is a weird value that doesn't work for batteries. I got his 10kHz oscillator yesterday but I don't know what I'm going to do with it yet.
The opamps will run just fine on +/-5V, so I guess the rest of it will be OK also. Since a 9V rechargable battery is anything but 9volts, I guess 5V is a good value to shoot for in the oscillator.
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