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Erik Mandaville

A new and very quiet DIY AE-1 preamplifier.

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It's finally finished, and the results are very satisfactory. I mentioned some time ago that I was in the process of a 100% rebuild, including a new and larger chassis, of my AE-1 preamp, and it's now thankfully finished. As good as I have always thought it to be in reproducing music -- fast, detailed, transparent, etc. -- the original AES version always had a problem with excessive microphonics and noise. I was never happy with it because of those characteristics.

I had on hand a partially punch chassis that was going to be used last year for a low-priced, dual-mono/dual-rectified stereo parallel feed power amplifier. However, with a little extra filing, drilling, and hole punching, I was able to use this chassis to make a new, less crowded version of the AES AE-1 preamp. Parts proximity were very close in the factory preamp, and they (AES/Cary) also had numerous AC connections very close to signal inputs/potentiometers/leads. Grid stoppers of several thousand ohms have also done wonders in calming the rather hyper active input grids in this circuit.

Also, this version of the preamp does not have a standby mode as the original. However, because it uses SS rectification and large filter caps, I am using a current limiter installed between the AC supply and high voltage transformer and separate filament transformer primaries. The current limiter presents an initial high resistance against in-rush, which then falls to a much lower level after several seconds. In essence, it's very much like the soft-start quality of a vacuum tube rectifier. I initially wanted to use a time delay switch, but found this other approach effective and much less costly.

The 100k ohm volume controls are way too sensitive, but that is easy to fix. The main thing for me was to quiet the preamp down. I'm using 12SN7s for their much lower cost over the 6 volt version of the tube, but still need to drop the DC heater supply about 1.5 volts. I also installed an extra stage of filter capacitance on both B+ plate supplies, which also seems to have helped lower the noise floor. Local ground nodes were used in all stages, and these are in turn connected to a single-point star ground. The chassis is hence connected to the circuit ground at only one point.

It now finally sounds very good, and I'll post a picture of it next week for those who might be interested. It's nothing fancy, and there are no wooden sides as there are on the original. Just a simple box with a straightforward and symmetrical layout of parts. I may jazz it up a bit as far as looks -- a couple of oak side panels would help it to visually match my amplifiers.

I have to admit that, as much music as my little Dynaco PAT 4 has given us, this new preamp, with its quad of 12SN7s, sounds exponentially better. The PAT 4 now sounds thin and shallow in comparison -- very much so, actually.

I'm also just about burned out from amp and preamp building, so I'm really glad this last project has turned out well! With a little extra tweaking here and there, I should be able to use my equipment to listen to music. It seems for months I have been doing things the other way around: Using music as a means of testing and comparing the different components in my system. That's something that has been bothering me.

Have a good Sunday,

Erik 1.gif

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Erik,

Very cool project. When you have a chance to get some pics posted, I'll sure be interested in seeing them. Yep, you are right about the difference in listening for work and just listening for pleasure and being. Enjoy!

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Thanks Eric. You must be feeling pretty good right now. I'll be interested in seeing your photo's too.

Edwin.

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That makes three of us (get the Nikon out and clean those lenses)!

It'll definately be much better to listen to the music again instead of evaluating equipment. But I was hoping you'd still be tinkerin' on everyone's tube gear awhile longer (you gotta mod my amp/preamp...Just kiddin', natch)!9.gif

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I've wanted to hear one of these AE-1 preamps for a while or at least since I lost 14.gif an ebay auction on one over christmas. It was brilliant custom made black chasis with a big voltage plate meter on the front. I think it sold for $400.

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I'll get a few pictures posted soon. I still need to find some knobs (what time does Radio Shack open!?), and lower the filament supply. Again, it's nothing fancy at all, since what I really wanted to focus on was performance. Audio aesthetics can be nice, but looks impart nothing to the sound. It's a Hammond (or Bud -- I can't remember)aluminum box with tubes, a transformer, and four black capacitors mounted on it.

Jim: I'd be happy to help you any way I can. I in fact have one last thing I want to do, which is build a DIY version of the Transcendent Grounded Grid preamp. I have one last empty chassis that is a good size for it, and it's something I can work on in the evenings and on weekends as time allows. I already have most the resistors and capacitors, but just need to buy the transformers for the neg. and pos. power supplies.

Erik

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Erik,

When you get the DIY GG done, that will make a most interesting compare and contrast report. I'll be looking forward to hearing that.

Also, as long as you are getting the camera out for the AES pre project, could you post some shots of your TT?3.gif

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I second most of this, except the potentiometer change, which doesn't seem to do anything useful. Major omIssion to me is reducing the gain, which is far too high at 13.5, and alone is responsible for much of the microphonics and instability. I'm planning to change this to about 5, by changing the feedback resistor to 33K. This might exceed the stability margin, so I might have to shunt it with some picofarads.

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Cool!  Amazing to see a response to something I posted fifteen years ago!  Erik M. and I are one and the same. 

 

...miss Dee...

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2 minutes ago, erik2A3 said:

..miss Dee...

Absolutely.

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Posted (edited)

Well the gain would be 13.5 if the 6SN7 grounded-grid section had infinite open-loop gain, but it doesn't. It has OL gain of about 10, given by Ra=100k/Rk=4k7, divided by 2 because of sharing Rk with the prior cathode-follower stage. So the actual gain is about 14dB. (The construction notes say 12dB which is not correct on calculation or measurement.)

 

However ... the actual problem seems to be that this unit has 6dB more gain from 10Hz downwards, for a closed-loop LF gain of 20dB, to which at present I am attributing the microphonics and acoustic feedback. Fortunately the cause is obvious , and the solution pleasingly simple. The feedback coupling capacitor is far too small at 0.1uF, and the solution is remove it and instead connect the 100k feedback resistor to the output side of the 2u2F output coupling capacitor. This basically flattens the LF response and also has the pleasing side effect of bringing that capacitor inside the feedback loop.

 

i will be trying this tomorrow and will report results.

Edited by EJP
Typo

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Posted (edited)

Sadly all that made no difference. I corrected the LF response even further by adding 1uF below the 8k shunt feedback capacitor, to reduce the gain to unity at DC and from about 10Hz down, which is how I think it should really be, but that didn't help either.

 

So I am presently considering the good old silicon rings around the 6SN7s. Curiously enough there is a brand of these called Cary ...

 

I also rolled off the HF response by adding 22pf (or possibly 220pF pF, can't remember) in shunt at the input, 10K grid stoppers between the volume pots and the V1 grids (instead of the wires, easy enough), and 15pF across the 100K feedback resistor, which should all help with the AM reception while still keeping it reasonably wideband at 100KHz (was nearly 1MHz). AM break-in only happens at the customer site so I can't verify that at present.
 

To document the gain arithmetic above: the gain of the grounded-grid stage is given by 100/4.7 ~= 21, divided by 2 because of sharing the cathode resistor with V1A, which halves the current and therefore the plate voltage swing, so about 10.5,  or ~20.5dB. This is A in the general formula for gain: A/(1+beta*A), which is used when where A is not large enough to ignore, so taking beta as 8/100 this yields closed-loop gain of ~5.7 or 15.18dB. This is still a little high for the customer's equipment, so I actually increased the 8K resistor to 10K, which yields gain of 5.14 or 14.24dB. All this needs to be corrected further for the two cathode-follower stages, one inside the feedback loop: I will update this post for that.

Edited by EJP
Further info

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I had to give you a like erik above for building that preamp. I saw your count was 0 with 4000 plus post. Show me some pics and I will give you another😎

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On 7/22/2019 at 3:55 PM, mr clean said:

I had to give you a like erik above for building that preamp. I saw your count was 0 with 4000 plus post. Show me some pics and I will give you another😎

 

Erik's account got messed up at some point, and he has been back on here as erik2A3. He's always been helpful and pleasant to have around here.

 

Bruce

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It's good to see updates on some of the old posts.

SSH

 

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Sadly the silicon rings made exactly no difference either, which some around me predicted. Maybe adding some damping material under the top, which acts like a soundboard, might help, or changing to some variety of 12A?7, which were built for low microphone. But my customer and I are just about out of patience with this unit. Shame in some ways, because I quite like the circuit and the sound.

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4 hours ago, EJP said:

Sadly the silicon rings made exactly no difference either, which some around me predicted. Maybe adding some damping material under the top, which acts like a soundboard, might help, or changing to some variety of 12A?7, which were built for low microphone. But my customer and I are just about out of patience with this unit. Shame in some ways, because I quite like the circuit and the sound.

 

How are you going to do that? You are talking about a a 6SN7 (octal socket) and replacing with a 12a?7 variant (Noval socket).

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