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Old 6th February 2009, 08:28 PM   #41
DaveM is offline DaveM  United States
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Now now... No need for cursing. I don't plan on using a board at all. Well, at least beyond perf board. I figure that if I get this done in a similar time frame to my F-4's I should be completing this in 2011. Certainly no need for jealousy here.

All I can say is build it. Use whatever pots you have laying around and listen to it while you wait for a proper attenuator. Here is a pic of my present preamp. It is a BOSOZ running off one power supply mounted to a Clementine box using a stereo pot on each side for each channel. Not quite enough balls for the F-4, but it gets the job done until I reach an Impass.
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Old 6th February 2009, 08:30 PM   #42
DaveM is offline DaveM  United States
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Here is the pre I was running. I used a small tackle case that I got free somewhere. Pretty funny when you know that I have a lathe and Bridgport in my garage and work as an engineer with full access to a machine shop. Get to work
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Old 6th February 2009, 08:35 PM   #43
Tea-Bag is offline Tea-Bag  United States
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Quote:
Originally posted by DaveM
Now now... No need for cursing. I don't plan on using a board at all. Well, at least beyond perf board. I figure that if I get this done in a similar time frame to my F-4's I should be completing this in 2011. Certainly no need for jealousy here.

All I can say is build it. Use whatever pots you have laying around and listen to it while you wait for a proper attenuator. Here is a pic of my present preamp. It is a BOSOZ running off one power supply mounted to a Clementine box using a stereo pot on each side for each channel. Not quite enough balls for the F-4, but it gets the job done until I reach an Impass.

Quote:
Originally posted by DaveM
Here is the pre I was running. I used a small tackle case that I got free somewhere. Pretty funny when you know that I have a lathe and Bridgport in my garage and work as an engineer with full access to a machine shop. Get to work


I try to re-use old boxes, old surplus heatsinks or whatever.
Call it green, call it cheap. Call it DIY.
The second PIC looks like a DIY maniac IPOD.
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Old 7th February 2009, 01:57 PM   #44
JimT is offline JimT  Canada
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AX finally arrived. LED's, Neon bulbs, Madia and CCS - lots of great features, enough that I'll have to reread it a few times for it to sink in. It's great to see that 2'nd harmonic distortion is so low at 20V output and 3'rd harmonic 26dB below that! I am still working on understanding the details so my first question is just a simple one. The power supply decoupling capacitor, C5, is attached directly across the output stage. Your schematic reminded me decoupling capacitors should be as close to the active bits as possible. Thanks for that. I had forgotten about that when laying out the PCB for a Szekeres style headphone amp I am working on. Finally to my question. You do not have a decoupling capacitor on the voltage amplifier section. Is this because the CCS "decouples" just fine without it?

Thanks
Jim
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Old 7th February 2009, 03:13 PM   #45
SY is offline SY  United States
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The cap serves to decouple both; their supplies are common. Morgan Jones mentioned in his book and in conversations that he had seen instabilities when the supplies to the classic direct-coupled-voltage-amp-split-load input stages in power amps were separated. Now with a CCS, I think that this would indeed not be an issue, but the CCS does indeed isolate that first stage from the supply. So why complicate things? A single supply to each channel is enough.

It was pointed out to me (and will appear in the next issue or two in the Letters section) by Chris Paul that the overall power supply rejection of the preamp is actually compromised by the second stage. Chris suggested that it could be improved by some noise injection into the first stage, and I agree with him on that. It might be worth some experimentation by someone curious, but since I'm seeing better than -100dB noise ref 20V, it's not high on my list of priorities right now.

Thanks for the nice words. It was a fun article to write.
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Old 7th February 2009, 06:50 PM   #46
BFNY is offline BFNY  United States
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Just read the aX article, one of the best tube articles there in a good while, congrats.
One comment/question. If you don't need a balanced input, would the input transformer be considered optional? With 1:1 It presents a 13K load to the previous stage, which some tube based sources may not like. Is this part of the magic?
Bob
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Old 7th February 2009, 06:55 PM   #47
SY is offline SY  United States
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It could be dumped, but I think the preamp is better with it unless all your sources are close by and you have well shielded cables and no ground loops. If you do it without the transformer, I'd put ground breaker resistors in the input jacks if you get any hum loops between sources.
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Old 8th February 2009, 09:02 PM   #48
BFNY is offline BFNY  United States
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Thanks for the comments on the input transformer.
The aX article references a previous article "The Heretical Preamplifier" from DIY Magazine with a longer discussion on the input transformer. The link provided, www.basaudio.net, seems to be dead today. But I found it on the Way Back Machine here
http://web.archive.org/web/200801170...net/diymag.htm

Another good read, thanks.
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Old 8th February 2009, 10:41 PM   #49
SY is offline SY  United States
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There's an updated version of the Heretical article on my website. Click the "www" button below, then it will be easy to find.
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Old 10th February 2009, 08:48 PM   #50
DaveM is offline DaveM  United States
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I am in the middle of putting together an order to Mouser and Allied. I was just wondering if there was any suggestions on cap selection. I have chosen Wima's and Rhodersteins in the amps circuit. In the power supply, I am curious if there is justification for using better than standard caps. I have been trying to use black gates where I can in other projects as the sound is dramatically improved. Also should I be building one power supply for each channels or is a single supply sufficient.

Also are the cap values in the power supply critical?

Thanks
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