Hi Everybody. I just wanted to fulfill my promise of coming back and letting you know if putting in lower gain tubes in my preamp would make a difference with the noise issue I had with the amp: sadly, no. But I found something peculiar when experimenting with a passive pre I have from 47 Lab. Turning up the volume increased noise until about the midpoint, and turning the volume fully clockwise made the noise go away. Very curious. As far as the tube preamp went, I was betting there was an impedance mismatch--not uncommon. But the passive pre/7297 behavior is surprising. Maybe the problem is how the amp was constructed.
Thanks for the offer of help. When I have a bit more free time, I'll dig in to the amp a bit and see if there is anything obviously wrong.
The passive has never given me any problems, though I prefer active preamps. Not mine, but it's this dead simple 47 Labs model 0447 assembled kit. Volume pot, input selector switch, RCA jacks, some wire, a case with bolts, and nothing else:
After eyeballing the innards of the amp and seeing that everything looked fine (no cold solder joints, loose connections, etc.), I managed to track down the noise/hum problem: the DC umbilical cord between the power supply and my preamp needed to be screwed in. Silly me, I thought that the cord just plugged in, but it plugs in and then gets screwed down. Live and learn.
It's still not an ideal setup because of the system's high gain, but that's no fault of the amp, which sounds very good, so I'm happy for now. I want good sound without having to turn on my First Watt F6 class A space heater during Austin's upcoming six-month summer.
It's still not an ideal setup because of the system's high gain, but that's no fault of the amp, which sounds very good, so I'm happy for now. I want good sound without having to turn on my First Watt F6 class A space heater during Austin's upcoming six-month summer.
It gets loud at about 9:00 o'clock--super early. I've ordered some 12-dB inline attenuators from Parts Express that I'll insert between the preamp and amp. I'm really enjoying listening to some jazz right now with this amp. I'm very happy with the sound.
I'll definitely be interested in future projects of this sort!
I'll definitely be interested in future projects of this sort!
With my speakers being nominally 97dB at 6 ohms, I'm more interested in lower-powered amps, which might be foolish of me, especially since I'm really enjoying the iron-fist-in-a-velvet-glove aspect of the 7297's sound. I'm expecting a Darling 1626 amp to arrive on Saturday. But I'll take a closer look at your latest offering.
I could still be running the the first LM3875 amp I made... but where would the fun be in that?
It's nice with DIY that you can build almost anything under $1k, and afford to try different stuff. That is opposed to buying something the price of a car each time, if you couldn't DIY. There's one dude on here that literally has boxes and boxes full of amp PCB's he's built. They probably span over a decade of DIYaudio... He's just hooked on building amps.
It's nice with DIY that you can build almost anything under $1k, and afford to try different stuff. That is opposed to buying something the price of a car each time, if you couldn't DIY. There's one dude on here that literally has boxes and boxes full of amp PCB's he's built. They probably span over a decade of DIYaudio... He's just hooked on building amps.
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