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Old 3rd March 2012, 01:29 AM   #31
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Here's a schematic incorporating one implementation of the "Full Monte" of revisions to increase bandwidth/gain and reduce distortion.There are other ways of going about this, of course.
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Old 4th March 2012, 03:00 AM   #32
BFNY is offline BFNY  United States
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While you are at it, what about a current source under the JFet input buffer?

Have you tested the stages individually? That could be a weak link. We know that the B1 implementation, for instance, or similar BF862 versions, both using 2 JFets, one as current source, the other as buffer, have extremely low distortion, i.e. ballpark under -100dB.
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Old 5th March 2012, 01:54 AM   #33
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Why don't you try this yourself?
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Old 5th March 2012, 02:14 AM   #34
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The first incremental change I applied to the circuit (actually a double change) was to add a cascode fet to the CCS drain load. The circuit now uses a DN2530N3 (TO-92) depletion mode fet for the current source and the original DN2540N5 (TO-220) depletion mode fet for the cascode. While I was at it, I reduced the current-determining resistor on the bottom device of the cascode to 61R9 from 100 ohms. This raised the bias current from ~20 ma to ~30 ma. The increase in the bias current improved the output centering.

The changes were beneficial, as evidenced for the distortion readings:
1kHz/0.1V - .014%, 1kHz/0.2V - 0.007%, 1kHz/0.3V - 0.005%, 1kHz, 0.4V - 0.0047%, 1kHz, 0.5V - 0.0047%

These numbers hug the residual readings of the analyzer for the most part.

10kHz/0.1V - 0.0153%, 10kHz/0.2V - 0.0143%, 10kHz, 0.3V - 0.0193%, 10kHz/0.4V - 0.0262%, 10kHz/0.5V, 0.033%

These numbers are an improvement over the earlier measurements.

While I was at it, I hung a probe on the output of the circuit with no input and detected a snarky little oscillation at a couple of MHz. This may be the reason why engaging the 80kHz low pass filter cleans up the distortion readings so much.
My next step will be to drag the thing off to work for a little square wave excitation and compensation work.
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Old 5th March 2012, 04:33 AM   #35
BFNY is offline BFNY  United States
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Quote:
Originally Posted by wrenchone View Post
Why don't you try this yourself?
Ok, I will when I have time.
My bench is overcrowded with another active project, and 2 waiting in the wings. And both bosses (the one at work, and my wife) just handed me more things to do, due yesterday.

Best wishes on the project.
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Old 5th March 2012, 09:20 AM   #36
MiiB is offline MiiB  Denmark
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would it not be possible to run it on +- supply and then DC couple it..??

Last edited by MiiB; 5th March 2012 at 09:31 AM.
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Old 6th March 2012, 02:57 AM   #37
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It would be possible to do what you suggest with an opamp servo - I wouldn't trust a passive adjust. I won't do it on the circuit I'm working with, as my test bed runs off a 40V supply with local regulation on each module to +30V.

Last edited by wrenchone; 6th March 2012 at 03:09 AM.
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Old 6th March 2012, 03:08 AM   #38
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I dragged the "fake SIT" in to work today for some square wave testing, which revealed some overshoot plus ringing at the edges - not too bad, but I don't like to see any. I put together a "gimmick" using some tightly twisted 26 AWG Tefzel wire (~3pF) and placed it across the 274k gain resistor. This changed my square wave response to overdamped, but a bit slow, so I lopped about !" off the gimmick. This sped up the output rise time to ~1usec, still overdamped. Declare victory! While I was at it, I added 10 nF bypass caps from return to chassis at the input and output connectors of my test box, and a common mode choke to the power leads consisting of about 5 turns through a medium-sized ferrite toroid. How all this affects the distortion readings will be revealed when I have some time to do the measurements.

Oh, yeah, and I added a 10k resistive load to the output. We'll see how this affects the THD readings. If you're looking for a preamp circuit that can drive a toaster, you won't find it here....

Last edited by wrenchone; 6th March 2012 at 03:15 AM.
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Old 8th March 2012, 05:15 AM   #39
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I was checking out a thread in the tubes section here regarding someone wanting a nice preamp circuit that could use a couple of 6CG7s he had on hand (nice tube). He was referred to a page on the Aikido preamp. I was looking at something similar for FETs (J and MOS). They're not totally applicable because of their generally pentodish nature, though some of the smaller SIT devices might work well in this application. At any rate, the usual jfets and mosfets yield up too much gain for use in the unadorned Aikido. What can one one do with all that gain? Well, you can feed it back. With that in mind, here is the result of some doodling around in PSpice. I omitted the RC circuit used for ripple cancellation - you can add it if you think your Vcc supply might be too ripply/ noisy. The circuit simulated as having a THD a little South of 0.001%. As per usual, simulations and reality may differ - I'm still struggling with the circuit that started this thread tying to get something that might approximate the simulated THD results. At any rate, it's more grist for the mill...
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Old 10th March 2012, 09:51 AM   #40
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One possible use for the circuit in the previous post might be to use it in an RIAA amp as an active equalization second stage. Circuit minus values (haven't figured them out yet) is shown here. A hint - LTSpice is free....
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File Type: gif New_Schade_Phono.gif (18.9 KB, 442 views)

Last edited by wrenchone; 10th March 2012 at 09:58 AM.
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