Preamp for both F4 pair and Adcom GFA-5300? - diyAudio
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Old 23rd September 2014, 09:45 PM   #1
Jeffs is offline Jeffs  United States
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Default Preamp for both F4 pair and Adcom GFA-5300?

Am I asking for too much from one preamp? Am I trying to use my welding goggles for reading glasses?
I realize simply pitching in $20 to support the forums doesn't give me priority boarding for stupid questions so hope it appears I have been doing my homework.
For the last month I have been using a 20w integrated NAD from circa 1984.
While I have started sourcing parts for a pair of F4 clones, I have picked up a used Adcom GFA-5300 for driving a new to me pair of 4ohm (89db? w/m) (again, circa 1984) speakers which my speaker repair person assures me spend a lot of time below 4ohm, or 2. I like to listen louder than average on occasion which is why I was considering running the F4's from a balanced preamp - but that probably isn't required - should it be I would like to be able to balance in the future. Generally I have always found I like a little more bass than amps I have use din the past provide. These were 1980's HK and similar time period NAD integrated units. I don't have a line out for external amps on anything I currently own so I need something and would like separate bass and treble if possible or settle on a single tone control.
The climate I live in would prohibit me from running a pair of F4's because of heat produced for most of the year. I often use AC in December... So Adcom GFA-5300 for most of the year, and the F4's for cold winter nights or critical listening with the AC blasting...
I don't need to be using the F4's for watching a football game.
I think I want something with tubes, but my budget is low.
Can I power both amps with the same preamp (even if switching tubes / maybe there is an easy power supply swap - but doubt it). Not at the same time, but switching cables over. A switch would be nice, but more pieces to introduce noise.

For the F4's - an Impasse balanced is often recommended. For the Adcom GFA-5300, that's throwing a lot of signal away. I suppose it could be done, putting an attenuator before the Impasse. A multi-turn pot is something I had thought of putting in so if someone were visiting then wanted to turn the volume up a little they would be less likely to make anything smoke. Maybe I could put something for tone control after it and before the amplifier.
I see variations on the Aikido that put out more gain, and even a single tone control circuit sold by one of the kit makers.

Ran across this on ePray, Tube Stereo Tone Control Baxandall Type Treble Bass Volume Using 12AX7 | eBay but have little info.
Have never tried SMT so avoiding it and associated kits, and a beginner at soldering. Have done a couple of 25amp heater controls, one triac, one PWM- but those were obviously not for audio.

Source will be FLAC files, may upgrade to a DAC but audio is either 24bit/192kHz or whatever a late 2013 Macbook Pro does. I'll need to make some accommodation for a line out from a television to be purchased this week, not sure what a $500 TV has for line out. It's possible that will require a DAC to be expedited.
Should I acquire vinyl one day I can worry about a turntable and the requirements for that in the future.

-Cheers
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Old 24th September 2014, 02:49 AM   #2
6L6 is offline 6L6  United States
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Are you listening to the Adcom right now? Or do you need a preamp to drive it?

Does the NAD have some RCA jacks on the back labeled "Pre Out"?
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Old 24th September 2014, 02:57 AM   #3
Jeffs is offline Jeffs  United States
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No pre-out on the NAD, I need a preamp to drive the Adcom. If that were the only concern I could find something on craigslist. Temporary solution vs desired one.
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Old 24th September 2014, 04:01 AM   #4
6L6 is offline 6L6  United States
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An Aikido would do the trick, it has enough gain, and for the Adcom you just wouldn't turn it up very much...

Still not balanced, so a pair of F4 would be 50wpc.
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Old 24th September 2014, 05:20 AM   #5
Jeffs is offline Jeffs  United States
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Working with partially completed BOM's at this site (which may not be affiliated) - Aikido Line-Stage Amplifiers they have many options and list tube combo's. Maybe 4-6 extra tubes to experiment with the best sound for different combo's? Can't decide on which one would be good for the Adcom, but with the best options for increased gain for the F4's (bridged?) - HPA options seem excessive, but I could be wrong.

Either a stepped attenuator before the preamp, or a potentiometer with more than one rotation of the dial to prevent a houseguest from making smoke on the Adcom and speakers. Not so sure balanced is needed for the first build, into 4 ohms a single F4 should do 50wpc, so 2x sounds like 100. I don't doubt that is enough but if I discover I'm $1500 in to the pre / amp combo, it's "only" balanced voltage gain to double output?
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