Adcom GFP-750

audiocr381ve

I don't know where the influence came from to spend all of my money (thanks guys), but I just pulled the trigger on an Adcom GFP-750. I'm going to audition it tonight. I wanted to check with my buddies (you) before I emptied my wallet and get some advice on what to look for etc.

The guy is local and is selling the item for his boss. It is the blue board version. He mentioned that a switch was replaced and the preamp has been serviced/cleaned. He said that the input switches do go out and it's a pricey service to have done. Is this true?

Are their things to look out for on this particular unit that may not be obvious?

heiney9

The input switches are not prone to go out because they are relay's. Because they are relays, which is a very expensive albeit best way to handle switching, replacing a relay can be a more expensive repair.

Which switch was replaced and why? The only switches are the power, passive and processor all other switching is done by relay. Many time people would break off the toggle switches because they are rather tiny and it's easy to snag or bump them. I know of no known issues with input switches (relays) so no need to worry about that.

Just make sure everything works

Refer to page 3 in the manual

http://www.adcom-usa.com/userguides/gfp-750-ug.pdf

H9

heiney9

Be sure it has a remote and it works. Last time I checked they were expensive to replace and it seems I read somewhere recently they are no longer available. You may want to check on that if it's missing the remote.

chandler9a

Let us know what you think of it. I am keeping an eye on one local to me as well but the auction is not up for a couple of days. Luckily I know the seller and have bought a couple of pieces from him in the past.

audiocr381ve

heiney9 wrote: »

The input switches are not prone to go out because they are relay's. Because they are relays, which is a very expensive albeit best way to handle switching, replacing a relay can be a more expensive repair.

Which switch was replaced and why? The only switches are the power, passive and processor all other switching is done by relay. Many time people would break off the toggle switches because they are rather tiny and it's easy to snag or bump them. I know of no known issues with input switches (relays) so no need to worry about that.

Just make sure everything works

Refer to page 3 in the manual

http://www.adcom-usa.com/userguides/gfp-750-ug.pdf

H9

Brock he said is was the input selector? He may be getting his terminology wrong. Anyway, thanks for the valuable info.

Chandler, I'll most definitely post a review in a few days. I have pretty high expectations as I'm finally getting into some better gear.

heiney9

Also make sure the shorting pins are there for the balanced connections. There are balanced inputs and outputs. Only the inputs need the shorting pins. If they are missing I have a source for them. You can't use the single ended inputs if the shorting pins are missing. There will be an impedance mismatch.

RickTfromAZ

My GFP-750 seems to have become ESD sensitive since changing from unbalanced to balanced IC's between the preamp and the amp. Occasionally now, when I touch the volume knob without first grounding myself, it will cause one or both of the following to happen: the Bypass will engage, and/or the source selector will change position. Not sure if anyone else is experiencing this problem or not. Possibly something to check for. I'd have it looked at if it weren't for it being replaced in a few days with a JC-2.

heiney9

Does it do it in the Summer when humidity is high? Or just the Winter when you are more likely to get/give a shock. I ground myself this time of year before touching any gear, because it can be badd juju if you don't.

H9

FTGV

audiocr381ve wrote: »

...he said is was the input selector? He may be getting his terminology wrong.

I believe it uses a rotary encoder( instead of a mechanical switch)that controls a logic circuit which in turn operates relays.

FTGV

RickTfromAZ wrote: »

My GFP-750 seems to have become ESD sensitive since changing from unbalanced to balanced IC's between the preamp and the amp. Occasionally now, when I touch the volume knob without first grounding myself, it will cause one or both of the following to happen: the Bypass will engage, and/or the source selector will change position. Not sure if anyone else is experiencing this problem or not. Possibly something to check for. I'd have it looked at if it weren't for it being replaced in a few days with a JC-2.

The static charge must be trigging the logic circuit.

heiney9

FTGV wrote: »

I believe it uses a rotary encoder( instead of a mechanical switch)that controls a logic circuit which in turn operates relays.

Correct, as I understand it, it's not mechanical at all.

H9

audiocr381ve

Should I clarify what exactly was repaired or should I see if it works and not worry about it?

heiney9

See if it works and not worry about it. All repairs should be clarified, IMO. But I did a lot of research on this pre over the years and I don't see any issues out of the norm for these.

The balance control is different than a typical balance control so it doesn't completley mute one channel when it's moved completely to one side. It works more like a gain device. That is normal.

After studying the various Aleph P schematics, I think the Adcom is virtually the same with just a few value changes. The Adcom has a potentiometer for volume instead of stepped attenuator, and has a balance control which is an elegent modifier of gain. The electrolytics in the gain network are omitted in the Aleph P. That's about it other than the cheap DC blocking caps on input and output.

Adcom's implementation of a balance control is interesting! It looks like they are adjusting the gain of the mosfets.


No Switching ICs or muting transistors, just relays in the signal path input and output. About 20 relays total in the unit!

Here is a good thread at DIY audio talking a bit about the design

http://www.diyaudio.com/forums/pass-labs/37960-replacing-adcom-gfp750-caps.html

H9

audiocr381ve

heiney9 wrote: »

See if it works and not worry about it. All repairs should be clarified, IMO. But I did a lot of research on this pre over the years and I don't see any issues out of the norm for these.

The balance control is different than a typical balance control so it doesn't completley mute one channel when it's moved completely to one side. It works more like a gain device. That is normal.

After studying the various Aleph P schematics, I think the Adcom is virtually the same with just a few value changes. The Adcom has a potentiometer for volume instead of stepped attenuator, and has a balance control which is an elegent modifier of gain. The electrolytics in the gain network are omitted in the Aleph P. That's about it other than the cheap DC blocking caps on input and output.

Adcom's implementation of a balance control is interesting! It looks like they are adjusting the gain of the mosfets.


No Switching ICs or muting transistors, just relays in the signal path input and output. About 20 relays total in the unit!

Here is a good thread at DIY audio talking a bit about the design

http://www.diyaudio.com/forums/pass-labs/37960-replacing-adcom-gfp750-caps.html

H9

Thanks dude. As much as I give you trouble, I take your advice way to often I wouldn't have made this purchase if it weren't for your high praises of it. Thank you sir. I believe it is a step in the right direction.

heiney9

Post #54 might explain what repair was done.

http://www.diyaudio.com/forums/pass-labs/37960-replacing-adcom-gfp750-caps-6.html#post1843281

It doesn't seem all that common.

My brother bought the one I have, new in box. He and I are the only owners and I haven't had an issue. I don;t move the selector ever since I only use the one input on the pre-amp.

H9

heiney9

The main thing is if you aren't using the balanced connections and the balanced input is missing the jumpers the pre-amp won't sound right using single ended (RCA) connections until you get the jumpers in place. You will have an impedance mis-match that will sound pretty bad. Muffled, lower output, etc.

Also be aware that there is quite a difference in volume level between passive and active so if you switch between the two, make sure to turn the volume down when going to active because you could blow or over extend your speakers if the volume is set too high. I suspect this is one of the reasons the remote doesn;t allow for active/passive switching. It would be way too easy to blow something.

H9

Gempler

Lots of good advice from all the above but I thought I would mention this to give you some comfort in buying if you have any doubts... There is a shop in NJ that specializes in Adcom repairs and the guy has a great reputation. Unlike the authorized Adcom repair shops that are slippery about what a repair might cost this guy offers flat rate repairs. For the GFP-750 his rate is $180 plus shipping. Considering the unit in mint condition goes for about $700 these days, you can knock off whatever you think is appropriate for the cosmetic condition, as well as $200 to cover a repair should it need one. Good luck.
http://www.ebcelectronics.com/adcom-gfp-750-Repair-Service.html

audiocr381ve

heiney9 wrote: »

The main thing is if you aren't using the balanced connections and the balanced input is missing the jumpers the pre-amp won't sound right using single ended (RCA) connections until you get the jumpers in place. You will have an impedance mis-match that will sound pretty bad. Muffled, lower output, etc.

Also be aware that there is quite a difference in volume level between passive and active so if you switch between the two, make sure to turn the volume down when going to active because you could blow or over extend your speakers if the volume is set too high. I suspect this is one of the reasons the remote doesn;t allow for active/passive switching. It would be way too easy to blow something.

H9

Gotcha. I do have some 20 footers laying around for vocal mics that I'll use for now. I was actually going to start a new thread to find some solid XLR's under $100.

audiocr381ve

Gempler wrote: »

Lots of good advice from all the above but I thought I would mention this to give you some comfort in buying if you have any doubts... There is a shop in NJ that specializes in Adcom repairs and the guy has a great reputation. Unlike the authorized Adcom repair shops that are slippery about what a repair might cost this guy offers flat rate repairs. For the GFP-750 his rate is $180 plus shipping. Considering the unit in mint condition goes for about $700 these days, you can knock off whatever you think is appropriate for the cosmetic condition, as well as $200 to cover a repair should it need one. Good luck.
http://www.ebcelectronics.com/adcom-gfp-750-Repair-Service.html

Good to know! Thanks for the link. Where can I find some references?

heiney9

audiocr381ve wrote: »

Gotcha. I do have some 20 footers laying around for vocal mics that I'll use for now. I was actually going to start a new thread to find some solid XLR's under $100.

You have a balanced source?

H9

heiney9

Here's the link to some very nice jumpers if anyone needs them. I bought a pair for my Aleph and some extras for a rainy day. Very high quality parts.

http://www.dedicatedaudio.com/inc/sdetail/115/5157

Gempler

All I can give you is mine. Shoot him and email or give him a call, but first call the authorized dealers so you have a sense of what you don't want :rolleyes:

heiney9

audiocr381ve wrote: »

Good to know! Thanks for the link. Where can I find some references?

They are on the website.

H9

audiocr381ve

heiney9 wrote: »

You have a balanced source?

H9

No but a balanced amp.

You guys have been a huge help.

acmf74

Speaking of jumpers... I have the 5802 using RCA connections.. My inputs on the amp dont have the jumpers... Do I need them on the amP?

audiocr381ve

acmf74 wrote: »

Speaking of jumpers... I have the 5802 using RCA connections.. My inputs on the amp dont have the jumpers... Do I need them on the amP?

nope

heiney9

acmf74 wrote: »

Speaking of jumpers... I have the 5802 using RCA connections.. My inputs on the amp dont have the jumpers... Do I need them on the amP?

It has a switch

heiney9

audiocr381ve wrote: »

No but a balanced amp.

You guys have been a huge help.

Ok, but the input needs to be jumped not the output, it's a moot point if the seller has the jumpers

FTGV

audiocr381ve wrote: »

nope

+1 it has a bal/unbal switch.

heiney9

FTGV wrote: »

+1 it has a bal/unbal switch.

Yep, I just looked at the owners manual. I guess I should have done that before.....lol

audiocr381ve

heiney9 wrote: »

Ok, but the input needs to be jumped not the output, it's a moot point if the seller has the jumpers

Ahh I didn't know that. I just text the seller and he does indeed have them.