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Old 24th October 2002, 07:52 PM   #1
Joel is offline Joel  United States
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Default My current 71a amp

This is the stereo I put together last month for my living room. Averaging about 30-40mW RMS when I listen, so at 3/4W per channel I have enough headroom to cleanly pass roughly +13dB peaks. No preamp needed, I go straight in from the CD outs.

Really great stereo image, with very non-directional bass. My girlfriend says it's creepy how much it sounds like the performer is in the room with you. Haven't put it on the scope yet, I've been too busy just listening to it, so I have no distorion figures or freq response - but it does sound very good. A definite upgrade from the triode strapped EL84's I was using before (Raytheon 5755 as preamp). The 6F5 is surprisingly dead quiet. At $2 each, you can't beat the price either, and the metal tube looks very 1930's with its grid cap.

None of the components are crucial, and no particular caution is needed with layout - it does not seem prone to oscillation, or hum. All-in-all a good project for somebody looking to build a very simple DHT amp for under a couple hundred bucks.

joel
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Old 24th October 2002, 07:57 PM   #2
Joel is offline Joel  United States
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Default ...and the PSU

Here's the power supply for it. You could also use a battery instead of the rectified bias supply.
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Old 24th October 2002, 08:07 PM   #3
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Joel,

Congratulations!


Cheers,
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Old 24th October 2002, 08:57 PM   #4
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Nice job...Joel..!!

There is something about a "simple" design that makes my heart beat faster...

Thanks for posting it...

Cheers and happy listening...Don't waste your time measuring it!!! You could spend that time listening';-)

Bas.
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Old 27th October 2002, 07:09 AM   #5
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Joel,

I like simple, and yours looks to be that.

I recall the fellow that designed the Darling say that he likes to have the power supply on his two stage amps in the same phase (ie tied to the B+ at the same place). I guess since the two stages are drawing power out of phase from each other the peak draw for one is the valley of the other. Anyways your amp would be an ideal cantidate for this. It would be interesting to hear your results if you try it. And the amp would become even simplier.

dave
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Old 27th October 2002, 07:47 AM   #6
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Hi Joel,

Congrats on the amp.
What speaks are you running with it that 3/4W suffices. Just curious because I've been clipping a 40W amp into my (modified) Khorns recently. Not often, and I have a large room but sometimes my neighbours like to share my music.

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Old 27th October 2002, 04:46 PM   #7
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Default SPEAKER QQ

Brett,

Joel provided info about his system in the indriduction section.

Altec 'Flamenco' speakers, 98.8dB sensitivity.

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Old 27th October 2002, 05:10 PM   #8
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Ta Frank,

I probably read that, but due to a....ahem....misspent youth, I have poor short term memory. Well, at least I think I do.

Oh well, if I can clip an amp rated 17dBW higher into 5dB more sensitive speakers, I guess I can cross the 71A off my 'potentials' list.

Cheerio
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Old 27th October 2002, 08:21 PM   #9
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Default 71A

Hi,

Joel,

Have you toyed with regulating (with tubes) the B+ of your amp?

Thanks,
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Old 28th October 2002, 02:04 PM   #10
Joel is offline Joel  United States
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Frank,

I haven't considered regulation, because a 71A running in class A only varies by a few mA across its input range. Even the scant regulation provided by the choke in the PS is really unnecessary in this amp - but it does help smooth the ripple. You could replace the choke in my schematic with a 100 ohm resistor, with probably no ill effects, or change in sound.
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