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Old 21st September 2013, 10:26 PM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Op-Amp Like Line Preamp

What do you all think of this design (see attached PDF)? It features:

- A loudness (volume) control.
- 4 inputs for separate sources selectable by a rotary switch.
- 6x 12AU7 type vacuum tubes (three per stereo channel).
- All direct coupled interstage circuitry.

I finally found in my 12AU7 collection a combination of tubes that yields low thermal noise. Replacing the blue velvet pot with an ordinary Alpha 50k made the preamp sound so much better. 9:00 on the volume is significantly loud for most recordings into a modified MKIII.

What surprises me most of all is how lifelike this preamp sounds. I have been listening to some compressed recordings and CD's and the preamp really brings out the harmony (even in MP3s). What makes this design sound clear and not muddy like more conventional topologies? Is it DC coupling?

Hope to hear back.
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File Type: pdf Line Stage.pdf (48.0 KB, 135 views)
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Old 21st September 2013, 10:45 PM   #2
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Could it be the positive feedback that is employed allowing for increased dynamics do you think?
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Old 22nd September 2013, 01:49 AM   #3
kevinkr is offline kevinkr  United States
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Yeah, I noticed that too, surprising it is not oscillating. To fix just swap the grid connections to the input triodes. (I've done this - in my case it was quite the oscillator.)
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Old 22nd September 2013, 01:59 AM   #4
BRSHiFi is offline BRSHiFi  United States
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I tested it and the feedback is all negative on both channels. When wiring, I by mistake miswired it and did oscillate like crazy. Fixed and all is good. Low noise, no oscillations of any kind, and very clean, detailed dynamics. I may have goofed the schematic. Will make a later revision if it contains an error.

Last edited by BRSHiFi; 22nd September 2013 at 02:01 AM.
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Old 22nd September 2013, 02:01 AM   #5
kevinkr is offline kevinkr  United States
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I've done that and surprisingly it radiated interference well into the VHF TV band.

Looks very well put together from the pictures, and I noticed the very clean electrical layout. Nice! I see so many projects both here and in real life that would be seriously embarrassed by that build quality.

Good end results often follow from the care taken in building, wish more people would figure that out.
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Old 22nd September 2013, 02:02 AM   #6
BRSHiFi is offline BRSHiFi  United States
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The miswiring on this unit caused a motorboat, no VHF. Where you doing RF?
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Old 22nd September 2013, 02:10 AM   #7
kevinkr is offline kevinkr  United States
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Actually audio interestingly enough, there turned out to subsidiary stability problems which became obvious later. This was very early in my sojourn into tubes, 12AX7s on a wood breadboard with long fly leads between the sockets and the interconnection grid had a lot to do with it. (Big screws with lots of nuts and washers for holding wires and parts) Grid stoppers at the sockets killed most of the problems with this thing. I still have it, never use it.. lol
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Old 22nd September 2013, 02:15 AM   #8
BRSHiFi is offline BRSHiFi  United States
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Quote:
Looks very well put together from the pictures, and I noticed the very clean electrical layout. Nice! I see so many projects both here and in real life that would be seriously embarrassed by that build quality.

Good end results often follow from the care taken in building, wish more people would figure that out.
Thanks a bunch. I have been through the movie many times of sloppy building. At best, the SN ratio will be horrible. At worst, the thing will oscillate like a rabid dog.
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Old 22nd September 2013, 01:58 PM   #9
DF96 is offline DF96  England
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Putting an active device in the feedback network is rather brave! You are relying on very good matching between triodes. Otherwise the feedback network could increase distortion. Scope for rather too many phase shifts in the loop, so HF instability could happen.

This is a good example of a circuit which will simulate much better than it works in real life.
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Old 22nd September 2013, 05:54 PM   #10
BRSHiFi is offline BRSHiFi  United States
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I did not match the tubes (it is true this design would perform better with it). The FFT registers some minor harmonic activity but did not get much of a THD fix.

Aside from that, I cannot hear any distortion whatsoever. The dynamics are detailed and rich. If distortion was a problem, how come it sounds good?
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