Adcom preamp overhaul vs. new A/B splitter + EQ vs. other options?

[thylipslip]

Hi all, first time poster here. First, my apologies for the LONG post!

I happily discovered this forum fairly recently while on the hunt for an audio community with a science based approach instead of the usual "magical thinking" that unfortunately seems to dominate most of the online audio discussions I've run across.

I apologize if this is a stupid question, but I've done a decent amount of research so far and have come to the conclusion that there are multiple possible viable solutions to my dilemma, but no extremely straightforward ones, hence... analysis paralysis. I'm hoping to crowdsource some of your knowledge and experience so I can make a decision.

I have a very basic, low cost system in our modestly sized family room with the following components:

• Adcom GTP-350 preamp (circa 1990s?)
• Audioengine 5+ powered bookshelf speakers
• Harmon Kardon FL8300 carousel CD player (circa late 1990s)
• Pro-ject 1.2 turntable (circa 2000?)
• BBE phono preamp (cheapo Guitar Center model)
• BluDento BLT-HD Bluetooth receiver

The preamp and speakers were purchased about 8 or 9 years ago. I specifically went with this type of arrangement due to space constraints that exist with the maximum height of my components. Anyway, I'm quite happy with this system for what it is, but the Adcom preamp has always been a bit of a nuisance. Due to its age, the pots are quite scratchy, which leads to the occasional partial cut out, mostly in the right channel. The balance and output selector knobs seem to be the worst culprits. I've partially disassembled the unit and sprayed contact cleaner in there, which helps somewhat, but it hasn't eliminated the problem. I believe replacing the pots is in order, which could be a bit problematic on the selector knobs due the the various detents present (when I researched this a few years ago parts for these seem to be a bit scarce, although still in circulation). Moreover, it looks to be a pretty labor intensive job, more than I personally would want to take on, even if parts could be sourced.

Something else that bothers me about the Adcom, and I don't know if this is unique to my specific preamp, or an issue with the GTP-350 in general, is what I perceive to be a high noise floor. There is a definite hum from this thing when just powered on; I'm assuming it's 60Hz hum, although I haven't measured it. It exists regardless of which input or output is selected; I've even messed around with the tuner and it doesn't seem to make a difference. It's only really noticeable when wearing headphones, so not a big deal, more of a minor annoyance. The old Denon DRA-435R integrated receiver I've had for over 30 years, which I use in my basement system, has a blissfully quiet headphone listening experience by contrast, so maybe I'm just spoiled.

So here is my dilemma: I've looked at replacing the Adcom with a new preamp and there just doesn't seem to be anything reasonably priced out there that has at least 3 RCA inputs and a headphone jack (preferably 1/4", although I could live with 1/8"). My definition of "reasonably priced" is around $300 or less.

There's some really sketchy looking stuff on Amazon at around the $100 mark (OSD and Pyle), but the quality looks highly suspect, not that Amazon reviews can be trusted at all. Emotiva has options starting at $500, then we get into $1K+ territory with NAD and others. One option that I've considered is the Aiyima T8. It technically only has 1 RCA input, but it also has a coax in (the FL8300 CD player has a coax out) and a built in Bluetooth receiver, which would replace my BluDento. However, I don't love the fact that it has a tube stage for the headphone amp. I wouldn't say it's a deal breaker per se, but it seems like an unnecessary gimmick. If it was totally solid state I would almost be sold, although again... quality??

This led me to consider other options. Since I never use the AM/FM tuner on the Adcom, I have thought about utilizing an RCA input selector switch plus an equalizer (I'd be happy with a basic 2-band bass + treble arrangement). But then what to do about the headphones?

I'm in the Chicago area, and there are a couple of places in town that will restore used audio equipment. I haven't inquired specifically with any of them about if they would work on the Adcom.

Am I missing something obvious here? Your advice is humbly welcome and much appreciated!

 

[Doodski]

The Adcom is a very nice preamp and possibly just needs some care and attention. I usually do NOT recommend using Nutrol to clean and lubricate pots and switches because it dries up after several months to a year and then the pots get stiff or scratchy sounding again. I have never seen a switch or a pot that Nutrol has not cleaned so that it was not scratchy. In this case you have a persistent difficult to clean pot and switches. So try some Nutrol cleaner and then after it dries up somewhat spray in some DeoxIT cleaner and lubricant. That should get things working.

 

[thylipslip]

I hadn't considered upgrading my contact cleaner. I'm using a can of Radio Shack stuff, which tells you how old it is!!

DeoxIT is an excellent suggestion and I think that's definitely worth a shot! Thanks for the advice.

 

[thylipslip]

As an addendum to this, I talked with a co-worker of mine who restores and resells antique radios (tube based). He has quite a bit of experience cleaning dirty potentiometers and he swears by DeoxIT F5, finding it to be superior to DeoxIT D5 in this particular application.

 

[Blumlein 88]

ADCOM GFA-535 II Power Amplifier & GTP-400 Tuner-Preamplifier Excellent | eBay

Find many great new & used options and get the best deals for ADCOM GFA-535 II Power Amplifier & GTP-400 Tuner-Preamplifier Excellent at the best online prices at eBay! Free shipping for many products!

www.ebay.com

For $345 you could get the GTP 400 and matching amp in the rarely seen white finish. Have to act in the next 12 hours. I have no connection to this ebay auction. The GTP 350 and 400 preamps go for around $100 to $150 if in good working order on ebay.

I used to have one of your preamp tuners. I seem to recall it had ribbon cables working everything at the rear, but don't remember about the pot. Have your tube guru friend look at it. Likely is nothing too exotic and he could get a similar replacement and put it in place. They do make detented pots that aren't terribly expensive.

EDIT: now that I think about it this had remote control didn't it. So maybe not a simple pot. I think mine was maybe a GTP500. Things you think you'll never forget, but do.

 

[thylipslip]

That looks like a reasonable price on the preamp / power amp combo, if purchased at the minimum bid. However, given the issues I'm having with my existing gear, I'm really not looking to take on yet another piece of 30+ year old equipment.

I had it open last year when I did my last clean of the pots, but I didn't take any photos. I do think I recall seeing ribbon cables for the LED display drivers on the front panel (or were those directly mounted to the main PCB??). My memory is terrible and even if I think I recall something I don't trust it!

My preamp doesn't have a remote, or if it does, it wasn't provided. I don't recall the volume pot looking like anything special, so methinks not.

Once I order the DeoxIT F5 and do another partial disassembly and cleaning, I will report back.

Speaking of disassembly... I'm pretty sure the previous owner had also taken this thing apart at some point because all of the screws were most definitely not in the right places. I know this because of the confusion I had during reassembly!

 

[Blumlein 88]

Looked at some pics of the front panel on the GTP350, and it did not have a remote as it does not have the little infra-red sensor on it. So that might mean it does just have a simple pot that can be replaced.

 

[Blumlein 88]

Here is a picture of one in the GTP 400 likely the same as yours. Courtesy of DIY audio forums.

Adcom pre-amp pot?

adcom preamp pot? Hi all, I'm the proud owner of an Adcom GTP-400 preamp-tuner with a funky volume control pot. I've removed it, but hours of paging through Mouser, Jameco, etc web sites has proven frustrating. I'd like to get a "drop in" replacement. I know ALPS is much better, but I...

www.diyaudio.com

The 4th lead was apparently for a loudness function. I think if you don't use that, any good 3 lead pot just ignoring the 4th lead would work. Alps are like $3-5 for one. Old one should have marked which value it is. I'm guessing 50 kohm based upon the original Adcom part number of V50KBX2 4TR-2586.

 

[MRC01]

Another good option for a vintage solid state preamp is the Rotel RC-990BX. I had one for years and preferred it to the Adcom, which was also very good.
That said, if a more potent cleaner does the job, then you're good to go!
BTW, I have an Adcom 5800 that still performs to specification, coming up on age 29 this year. The only thing I've had to replace over the years is the power switch, which lasts for 5-10 years at a time. At least some of the Adcom gear has excellent longevity and durability.

 

[thylipslip]

Well, the volume pot hasn't given me any trouble. It's the input source selector and ocassionally the balance control that act up from time to time. The input source selector has 4 positions with detents. That'll be the tricky one to try and replace. Crossing my fingers that a good cleaning does the trick.

I believe I might have looked at that Rotel back when I bought the Adcom. I think I would have been happy with it as well. Nice that it has the built-in phono stage.