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Old 22nd July 2015, 02:59 AM   #1
bogie71 is offline bogie71  United States
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Default Pre amp for $1000.00 ???

Hi i am currently using a Adcom GFA 555 amp and Adcom GFP 565 pre amp and i want to upgrade the pre amp. I was thinking about the Adcom GFP 750 i can buy one for $750.00 and have $250.00 left out my $1000 budget but would have to buy a phono amp to play my vinyl. The guy that recapped my GFA 555 keeps telling me to get a good tube pre amp like a Dynaco pas3 and make some popular mods thatís $1000.00 bucks and it would have a upgraded phono stage. Or is there any other highly regarded solid state pre amps i should consider ? what about some of the mods that Big sky audio or musical concepts are doing to the GFP 565 are they worth it ???
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Old 22nd July 2015, 04:17 AM   #2
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For $1000, I'll go to stores and listen to different ones before making the decision. I have not been in the market for a long time, BUT $1000 for a preamp without phono is a lot of money.

Now a days, the CD, DVD players can literally drive the power amp with a volume pot. I am building a preamp with just switching relays to choose between 3 input sources, then drive a volume pot. I use a LM4562 buffer to buffer the output of the volume pot and drive the power amp!!!

Last edited by Alan0354; 22nd July 2015 at 04:23 AM.
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Old 22nd July 2015, 08:03 AM   #3
bogie71 is offline bogie71  United States
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Unfortunately i don’t have any stores in my area that sell good 2 channel gear and i need the phono as i do most my listening on vinyl and rarely on cd. Ben doing some reading online and think i am going to buy the GFP 750 pre amp so i guess i need to start looking for a good phono amp.
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Old 22nd July 2015, 06:55 PM   #4
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The GFP-565 basically is a good preamp. If I had to find any negative, it's that the good folks at Adcom were obviously more concerned with low distortion than with lowest noise at the time, and so the main voltage amplifier stage is rather run of the mill in that regard - the responsible opamp (IC201/202, ADCOM6A = LT1056) is rather on the noisy side, and gain isn't exactly low at almost 20 dB. If the amp is true to its specs, after the GFA-555 with 27 dB of gain you're looking at about 450 ĶV of output noise voltage now. Which is rather mediocre. It may not even bother you when you've got power-hungry, insensitive speakers in a well-treated room... but otherwise it very well might.

I'd propose swapping the opamps in question for nothing less than the venerable OPA627 (modding the circuit to work with a bipolar part would involve some hacks to the circuit board) while downsizing R212 and R214 by about a factor of 4 and adapting / adding to C224 for stability if need be. Rinse and repeat for the other channel (IC201, R211/213, C223). While not cheap (genuine OPA627s run about $20 a pop), a mod like that should still run you about 10-14 dB less than $750, labor included. That's quite a lot of vinyl.

The headphone output also is rather hot by today's standards, bringing total gain to an estimated 26 dB or so. I'm not sure whether putting a current limiting resistor in the feedback loop is entirely beneficial to stability either (headphone cables may provide up to 1 nF of capacitive loading), especially with no extra local decoupling in sight. Output impedance should be very low, which is basically commendable but would make the potential output noise problem even worse. For a start, I'd suggest swapping R301 for R303 and R302 for R304, and vice versa - that should bring down levels (and noise) by about 7 dB. Feedback resistors R305-308 would also seem to be higher than they would need to be, I'd suggest bringing those down by about a factor of 3 to 3k3 (R305/306) and 9k1 (R307/308), respectively. It's not like a '4556D couldn't drive them! Plus, better impedance balance. This would leave gain unchanged - reduce it at your own peril (capacitive load testing recommended).

Other than that, the phono input (which is not as extremely low in noise as the ADCOM8A = LT1028 would suggest, but still very good) has a fixed 47 kOhm || 100 pF input impedance with no adjustments. If your cartridge is happy with that + cable capacitance, fine, otherwise you can only vary capacitance via the length / type of cable, and maybe make an adapter to put resistors in parallel externally. People often have trouble matching the C requirements of AT cartridges, for example, and then like to drop R to 36-39k (which evens out the response at the cost of a less extended frequency response up top).
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Old 22nd July 2015, 08:46 PM   #5
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You're in the southeast. I'd say look around for an old Peavey disco mixer (the one with the RIAA input) then change the op amps from 4580 to something quieter hiss wise. 4580 has enough drive to run a 100' twisted pair from mixer to stage, but you as a homeowner won't need that much cable load (op amp current out).
I've seen pictures inside numark and behringer mixers, the ones I've seen had ribbon cables and surface mount parts. Ribbon cables have punch in connection, which is not gas tight in the long run and may drop out, lose connection. My old PAS2 preamp has soldered connections and a plated switch so it percs along fine, as long as you don't mind burning 100 watts all the time. Surface mount parts IMHO require a $700 workstation and microscope glasses to modify. During the disco era when Peavey was making disco mixers (nineties) they were still using leaded parts you can change with a $40 WP35 iron and $1 reading glasses.
I lost some paper caps to moisture in the PAS2, and the plastic replacements were too trebly. (I use piano LP's and CD's as sound tests, a piano sounds like itself or it doesn't. I have a Steinway as calibrator between the speakers). So I changed op amps in a $15 flea market herald electronics RA88a disco mixer to eliminate the hiss. 4558 to 33078. The packaging of the RA88a was done without consideration of hum, so I took the transformer and power switch out (right next to the high gain stages, stupid) and replaced them with a wall-transformer. Much better, RIAA freq response is better than the re-capped PAS2 (to my ear) and the hum is almost as good as the PAS2. Even better, it draws 10 watts instead of 100. I deleted the dynamic voice mike input , and reused the pot to control a 7 VAC FM radio input. Great slide pots in that little mixer, no dropouts. The rotary pots in my 1996 band refugie Peavey Unity mixer are good too, but it doesn't have an RIAA input. $15 for the used RA88a and $20 in parts, quite a deal for decent sounding input selector and 50x gain on the mag phono cartridge. A winter of fooling around, but it was very educational. See this thread, especially near the end where things worked better. Improving a "Disco mixer" to mid-fi performance
I can't see paying $1000 for a used preamp that doesn't have RIAA inside. Like the man says, the Peavey CS800s power amp has enough gain on the input stage to run from a CD player, radio or computer input. What is cool about my disco mixer, I can leave all three slide pots (CD, radio, RIAA) turned up and just turn on which source I want to listen to, without walking around the table to switch the input on the PAS2 preamp when I change from one to the other. The RA88a hisses less than the pilot light on the gas stove.
Have fun buying used junk pigs and making pegasus out of one. It's do it yourself audio, after all.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 22nd July 2015 at 08:57 PM.
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Old 22nd July 2015, 09:22 PM   #6
rayma is offline rayma  United States
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Originally Posted by bogie71 View Post
Or is there any other highly regarded solid state pre amps i should consider ?
How about a dedicated phono preamp and a passive preamp (volume control in a box)?
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Old 22nd July 2015, 11:54 PM   #7
adason is offline adason  United States
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I have got few old NIKKO preamps, and Onkyo Integra 306 or some similar, all in the vicinity of $100 each from fleabay, after some cap upgrades these are pretty amazing.
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Old 23rd July 2015, 02:12 PM   #8
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Best preamp I've heard.
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Old 24th July 2015, 04:02 AM   #9
djoffe is offline djoffe  United States
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Default Build it!

The Akitika PR101 Preamp kit:

PR-101 Stereo Preamplifier

Just $450, with remote control...add another $74 for the phono preamp section.
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Old 24th July 2015, 06:40 AM   #10
Hikari1 is offline Hikari1  United States
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Since this is diyaudio I would strongly suggest you look at some of the popular offerings here...

Maybe the pass BA-3 preamp, the Salas phono or Pearl 2 or even the Hagerman bugle for phono. All quality stuff that can be built in your price range and that would be gear that retails for thousands of US dollars.

I've been through the retail stuff in your price doesn't compare to DIY for the same money. If you need remote controls, fancy cases, etc. you may want to look at retail stuff but really, it pays to DIY.
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