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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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In Reply to: DC offset posted by George Kubitsch on November 12, 2001 at 01:17:12:
I thought of using the MP-1's servo circuit over a year ago and kludged something up on the bench to test this idea. One of my "MA-30s" was used as a test bed. I took it one step further and essentially used a version of the MP-1's circlotron output buffer to drive the output section. The original V4 driver circuit was not used. So, I had a servo controlled driver circlotron driving an output circlotron. The servo's loop sampled the offset voltage at the speaker terminals. After a good deal of part tweaking, and the use of 5687s instead of 6SN7s in the driver stage, it actually worked rather well. Offset voltage was kept within about +/- 20 mV after about a 20 minute warmup.So, to answer your question about long-term bias stability, the circuit did nothing to improve long-term stability. All it did was obviate the need to adjust offset voltage over time. Bias was still an issue. If an output tube or two died, it could not adjust the offset, an error LED came on, and I knew it was time to find a bad output tube. It took a good deal of time for the servo to settle, compared to an MP-1. It takes a 6AS7G much longer to warm up and stabilize, compared to the 6SN7s in the MP-1.
Why didn't I take this project further? It added a good deal more complexity and required another tube. The added complexity reduced reliability IMHO. The fact is, the stock Whetstone bridge driver is very reliable and quite stable. IOW, as is, it works fine as is and sounds very good. Now a circlotron output stage driver, instead of the factory V4 circuit does hold promise. Some day I'd like to experiment with such a circuit.
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Follow Ups
- Re: DC offset - mark 08:09:40 11/15/01 (3)
- Re: DC offset - George Kubitsch 01:11:06 11/16/01 (2)
- Re: DC offset - mark 13:10:53 11/16/01 (1)
- Re: DC offset - George Kubitsch 03:01:03 11/19/01 (0)
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