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
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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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
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
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.
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.
Blast from the past....I just built this simple 71A circuit from my spares stash really because I'd bought a cheap lucky dip box of tubes on eBay which to my surprise also included a 26, 71A and cx380 rectifier which I was looking to use some of and as I recently switched to Lowther Acoustas and a sub I wanted to see how flea power I could go. The build was on a surplus power supply chassis I sprayed blue, existing holes limited the layout a bit. As the power tx wasn't bought specifically I added a 5v tx for the rectifier and another 6.3v for the 6FA driver filaments. I also separated the bias and filament supplies for each channel. The amp works really well apart from 2 things I need a steer on. Firstly the 500k volume pot is acting like a hum pot with low hum at either and and progressively louder hum towards the middle. I'm running it with the pot full on as I'm using a preamp. The other thing is although the sound is very warm and generally very nice, the really high frequencies get annoying after a while like there is some very high frequency distortion and no it isn't the Lowthers....I swapped back to my Morrison micro 2A3 and while it is very neutral it doesn't have this problem on the lowther pm6A's. Is it likely to be caused by the el cheapo Xse output transformers or something else? The outputs are biased at 20ma which is recommended on the data sheet. It's only my second point to point amp and my first with fixed bias. I'd like to try and resolve these issues as it's got potential!
Mike
Mike
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