Polk and ADCOM - a marriage made in audio heaven?

250GTLusso

I am starting a thread here so we can jointly learn from everyone's experiences with using ADCOM amplifiers and our beloved Polk SDA speakers.

Over the years I have acquired the following ADCOM equipment (due to their common ground configuration):
1.) GFA-555 (original Nelsen Pass design)
2.) GFA-565 (stock pair)
3.) GFA-565 (pair shipping this week to Big Sky Audio for their Upgrade)
4.) GFA-585 (stock)

I use the above amplifiers with the following pre-amplifiers
1.) GFP-555 (stock)
2.) GFP-565 (stock)
3.) GFP-565 (shipping this week to Big Sky Audio for their Upgrade)

I have used the GFP-555 and GFA-555 with my Polk SDA 1C's for 25+ years, and the GFA-555 does everything well, and most things exceptionally well. It provides superb, well-controlled bass. Its midrange and treble are remarkably low in coloration. There is no hint of hardness. As a result it has been the work horse in my rig for decades (which originally started with a Hafler DH220 amplifier and Hafler DH110 pre-amplifier when I first graduated from college - both made from kits of course!).

I have recently acquired 2 pair (yes 4) of the GFA-565 amplifiers. 1 pair in good working order with the capacitors having been replaced years ago, and another pair with excessive DC offset and in need of having their original capacitors replaced (I am shipping these, along with on of my GFP-565s to Big Sky Audio this week for their audio upgrade). I have yet to listen to the GFA-565s due to concerns with how to hook them up with my Polk SDAs (common ground concerns, etc.). This weekend I received a call back from the ADCOM USA service technician regarding the proper configuration for the GFA-565s with the Polk SDA SRSs. His recommended set up with different than I have read here on this forum, so I am going to move forward with caution and report back to him with feedback.

I have also acquired a GFA-585 Limited Edition. I just recently unboxed the GFA-585 LTD, and I am in the process of integrating it into my rig for an evaluation as this is a common ground amplifier and should be a significant upgrade over the GFA-555.

Over the next several weeks (and months) I plan on trying the various combinations of the following:

Polk
1.) SDA-2B
2.) SDA-1C
3.) SDA SRS (stock)
4.) SDA-SRS (modified)
4.) SDA 2.3TL (currently stock, but planning upgrades)

ADCOM
1.)
a. GFA-555 (original Nelsen Pass design)
b. GFP-555
2.)
a. GFA-565 (stock pair)
b. GFP-565 (stock)
3.)
a. GFA-565 (Big Sky Audio Upgrade)
b. GFP-565 (Big Sky Audio Upgrade)
4.)
a. GFA-585 (stock)
b. GFP-565 (stock)

Interconnects
1.) MIT Shotgun S3.3

Speaker cables
1.) MIT Bi-wire2

If any of you have experience with any of these combinations, I would appreciate hearing your thoughts and your learning's!

Peering into the rabbit hole.....

Nightfall

250GTLusso wrote: »

This weekend I received a call back from the ADCOM USA service technician regarding the proper configuration for the GFA-565s with the Polk SDA SRSs. His recommended set up with different than I have read here on this forum, so I am going to move forward with caution and report back to him with feedback.

Well don't stop there, what did he say? I find it hard to believe he knows something this forum and Polk themselves don't know.

250GTLusso

The ADCOM service technician (he covers all of the Legacy ADCOM components for ADCOM USA) researched his literature (300,000+ pages of ADCOM service bulletins and schematics) and said that the GFA-565 is common ground, and that the negative binding posts are already connected to the individual amplifier chassis ground, and that chassis ground is connected to the ground prong in the power cord, and therefore the two amplifiers have the same ground plane through my power conditioner, so you don't need to strap the negative binding posts, or connect the chassis of the two amplifiers together with a jumper. In fact he advised against it as it could create a ground loop that could induce an audible hum.

This is contrary to what I have read on this forum, so hence my skepticism.

F1nut

Having two mono blocks plugged into the same outlet doesn't make them common ground.

motorhead43026

Wow, researched 300,000 + pages of literature and still got it wrong.....lol.

leftwinger57

I've had my Adcom GFA-555 original rack face model now for about 6 years and got it from Audio Classics sound and sight unseen. Just on their reputation and their very conservative rating system. Mine came speced @ 205 wpc and some say it's a bright sounding amplifier. For my ears and never owning another amp I'm not all that sure. With the multiple pre/amps I have gone through my favorite and newly bought NAD 1020a is a great match and now I can tell it does take the edge off. It weighs about 60 lbs.

and has the toridal x-former and mine is clean in side and out. Oh, I got mine for exactly $300 and they threw in shipping for free. I also got the NAD pre from them.
No affiliation just a customer 2x over.

FTGV

F1nut wrote: »

Having two mono blocks plugged into the same outlet doesn't make them common ground.

In the scenario described by the Adcom tech it actually would.However a quick look at the schematic for the 565 looks like there is a 10 ohm resistor connecting signal ground to chassis ground,not a direct connection.The resistor is there to prevent ground loop issues while still leaving a path to safety ground.If the schematic is correct then to make the two amps grounds truly common a shorting link between the negative terminals is required.

Schurkey

FTGV wrote: »

F1nut wrote: »

Having two mono blocks plugged into the same outlet doesn't make them common ground.

In the scenario described by the Adcom tech it actually would.However a quick look at the schematic for the 565 looks like there is a 10 ohm resistor connecting signal ground to chassis ground,not a direct connection.The resistor is there to prevent ground loop issues while still leaving a path to safety ground.If the schematic is correct then to make the two amps grounds truly common a shorting link between the negative terminals is required.

Adcom has a history of "forgetting" that there's a resistor between the negative terminal and chassis ground; or they can't understand that that resistor plays hell with SDA speakers. My amp also has 10-ohm resistors between each channel's negative terminal and ground, 20 ohms in total between the two channels. It made my SDA 1Bs unlistenable. Another member with the same amp had 3.1TLs, and had actual noise and amp shutdown situations. Instantly fixed with eight inches of primary wire and a couple of banana connections. You'd need more than eight inches to couple the two amps, but the principle remains.

Of course with 1Cs or other pin/blade SDAs, you have the option of an isolation transformer--but that is not compatible with my 1Bs, or your SRS if they're the original edition.

I have personally sent Adcom e-mail telling them that they're screwed-up; the resistors between negative terminals and chassis ground will NOT work with SDA speakers, but either the guy I sent it to has been replaced, or he is simply ignoring facts.

250GTLusso

FTGV, and Schurkey, thank you for your insight into this interesting situation.

I have attached the schematic for the GFA-565 below. Can someone point out the 10 Ohm resistor in question?

F1nut

I'd get a Dreadnought and worry nought because from what I've heard with my ears, strapping has a negative effect on the sound quality.

F1nut

I can't read the schematic, too small and your host site has a virus.

Schurkey

F1nut wrote: »

I can't read the schematic, too small

Yup.

FTGV

Upon closer inspection it appears to be 100 ohm not 10 and has a 1uf cap in parallel with it.See section Z018A R803.

250GTLusso

That 100 ohm resistor along with the 1uf cap is a noise filter to reduce the hum from the chassis to speaker ground.

250GTLusso

Apologize if Photobucket isn't working properly. When I click on the picture in my post it takes me to the full size schematic.

250GTLusso

FTGV, that 100 ohm resistor (Z018A R803) is on the positive binding post, not the negative binding post.

FTGV

Well the schem I'm looking at it appears R803 and C805 are connected between circuit ground and chassis ground.Z018A appears to be a small PC board they are mounted on.

250GTLusso

Here is a close up of the GFA-565 outputs

FTGV

That's the power supply filter capacitor section and AC input not the output.You can see one end of C805 and R803 are connected to center or ground point between the filter caps.The other end is connected to chassis ground where you see the ground symbol.

250GTLusso

My apologies for the sloppy crop job FTGV. This view shows the GFA-565 outputs (the 4 binding posts shown in the upper right portion of the image below) with the path to chassis ground. The top and third one are the positive and the second and fourth are the negative.

FTGV

While it shows the line from the negative binding posts passing through the chassis ground line,the actual connection point is at the center ground point between the main filter caps c801 c802.Connection to chassis ground is through R803 C 805.

FTGV

If you have a multimeter with the amp unplugged try measuring resistance between negative binding post and ground lug on end of AC cord.

Schurkey

FTGV wrote: »

If you have a multimeter with the amp unplugged try measuring resistance between negative binding post and ground lug on end of AC cord.

That'll do it. You'd be looking for the lowest possible resistance; less than one ohm for sure, because whatever you measure on one amplifier will be doubled with the second amplifier, plus the resistance of the connection through the power strip.

Testing all of that would involve plugging both amplifiers into the power strip, then measuring from a negative speaker terminal of one amplifier to a negative speaker terminal of the other amplifier. I'd expect that the amplifiers would not need to be powered up, in fact the power strip could be turned off.

I guarantee you I'd rather poke 12-gauge primary wire and banana plugs into the negative terminals to strap those two amps together than to deal with the power cord and the power strip as a common-grounding method. I would route the 12-gauge wire well away from the power cord to avoid 60-cycle hum.

FTGV

According to the schem I would expect him to measure the 100 ohms of R803 between the ground lug and negative binding post.Assuming so strapping would be required and would not introduce a ground loop.

250GTLusso

The 100 ohm is a bleed off resistor to make them both equal potential to prevent humming. I am going to take a multi-meter and take resistance measurements and report back.

250GTLusso

Well I haven't had the chance to check out the stock amps (too busy with work and preparing for Christmas).
So in the mean time I am shipping off my other set of GFA-565 monoblocks and GFP-565 pre-amp to Big Sky Audio for their upgrade.
Anyone else have this done?