2. amps-preamps
  3. 1094084808

How to bridge a Amp Adcom GFA1A and run a Subwoff

Many of you will think "God this guy is a IDIOT" for asking this question. But been away from this hobby for 14 years.

I know that most AMP's that where made to bridge have a switch on the back for this. I have a old ADCOM GFA1A that does not. Can this still be bridge? If so how?

Little thinking out loud here, so maybe way off. If I where to split say the left channel to where it has 2 + and 2 - RCA plugs, then plug one set into the left input, and other set to the right input would that Work? If not, with ou hurting my head, why not?

Trying to drive a pair of DQ 10's and GFA not up to it. I did buy a ADCOM 555, not sure it will do it alone. Looking for a second 555 now,but in the mean time I have the GFA to play with. So what the heck.

I had not fired up the system for 8 years. Dragged the AMP, Pramp and CD player out to the garage where I could work on it. Worked out the bugs (dust in the preamp) and got the system going. Been useing a old set of Panasonic SB-750's that I might add sound pretty good for 36 year old speakers. Dad bought them when I was 12, Grank Funk sounded great on them when I was 14. Pulled one of the DQ's out, thermal shut down at 1/3 volume. Yep, I remember why I lost intrest.

I was also thinking about run a subwoffler, how? Peamp is a old Adcom GFP1A. When I get the second 555, will the GFA1A work to power a sub?

One last dumb a-s question. I broke the speaker cable off one of my DQ 10's. Pulled the wollfer so I could repair the it. Now why in the heck do we run monster cable's to the speakers, and on the other side of the termial the wires are 16 to 18 gauge? Look's like the speaker wire's I used for my 8-Track player in my Road Runner when I was 17. I might add I still have the 68 Road Runner, 8-Track long gone.

Cuda Ken
cudaken
7 responses Add your response
sean
6,231 posts
 
The GFA-1 is not bridgeable by design. That's because each channel already consists of two amps bridged together to make one bigger amp. As such, trying to tie both channels together would in effect be trying to tie four amps together.

What you can do is to have someone convert the amp into a mono-block. That is, take the power supply from the left channel and run it in parallel with the power supply feeding the right channel. While you lose one of the channels and the power isn't doubled to the one that you have left, it does "stiffen" the supply very noticeably. The end result is that you drastically increase bass slam and control.

As to the amp going into thermal shutdown, is the fan working? I've used these amps before and you can throttle them quite hard for extended periods of time without much of a problem. My guess is that there is something wrong with the amp due to sitting unused for so long. Capacitors may have dried out, the bias may be out of whack, the fan could be dead, etc... Sean
>
tripper
223 posts
 
I think it was Parasound that once advocated running just one channel of a stereo unit. Though you're at half-power, the power supply runs only one channel. Perhaps Sean can comment, I was an English major!
cudaken OP
6 posts
 
The fan work's on high and low speed. I thought the thermal shut down had a problem as well. Sent the GFA1A to a local high end repair shop to be tested, they could not get it to shut down? By the way, it had the problem the day I brought it home, it was 90 day's old then. I bought it from a guy that used a in store credit from a local high end shop. He could not pay for it and had to sell. When Brain and I went to listen to the system we wanted to crank it. But the seller lived in a apartment and could not let us. Well that sounded right, it was 9:30 went I bought it. Got it home and guess what happen, shut down.

Sean said "As such, trying to tie both channels together would in effect be trying to tie four amps together."

OK, like I said I am dumb, what would happen if I did? Go BOOM, light up the wireing in the houes, make the earth stand still or just sound like crapp?

I don't know stuff about this hobby but spend. But I am a gear head, I can bulid a 10.0 car (drag raceing 10.0 is zero too 139 MPH in 10 seconds) with a budget that it cannot be done with. Detail would help, in this hobby I could not set points or change plugs. ;-)

On the converting to a mono-block, any projected cost on this? Well that is a idea I like, that would help if used as a Sub Amp was well.

Speaking as a sub amp, how do I do that?

Cuda Ken
kgveteran
377 posts
 
I use a slew of GFA 555mkII's in bridged mode on 4ohm loads with no problems.
sean
6,231 posts
 
Kg & CudaKen: The GFA1 & 1A are VERY different amps from the 555 series of amps. I can guarantee that if the amp was bridged, it wouldn't be an amp for long. It would be a smoking metal cube that was on fire. This amp can't even be connected to a commonly used "demonstrator switchbox" that many dealers used to use. That's because the grounds are common between the channels and it will send the amp into meltdown.

As a side note, you can download the manual for this amp on the Adcom website. It doesn't have much info in it, but it does specifically state that you can't bridge it.

As far as "doubling up" the power supply i.e. converting it to a mono-block, i have no idea what someone would charge to do that. If you could get the schematic from Adcom, the time & labor charges would be DRASTICALLY reduced because the person doing it wouldn't have to sit down and figure out what was going on where. They could look at the schematic, make notes and dive in. As such, that might be where you want to start i.e. calling Adcom for a copy of the schematic. Sean
>
cudaken OP
6 posts
 
Sean, thank you agin for your time. Smokeing black cube I can understand. When you say the left and right side share the same gound you mean the power supply, right?

But my first idea, say take the left side from the peramp, split the line before the Amp. In other word's make 2 + and 2- set of wires. Run one set to the left channel, and one set to the right channel of the GFA1A. Tie the left and right wires too the spaeker before the speaker.

In other words, trying to fool the AMP into thinking it is power two driffrent channels. God must seam like a block head to the people that know what they are doing hear.

Any answer on what I need to do to run a sudwoofer?

Plus bounes dum a-s question. I have one 555 Amp, does the second Amp have to be a 555?

Dumb Cuda Ken
sean
6,231 posts
 
You can't connect the output of each channel to one common driver. You can use one channel to drive one woofer and the other channel to drive another woofer, but not both channels to drive one woofer. That is, unless the woofer is a dual voice coil unit and you're running them in parallel.

As far as running two different amps, this can be done, but you can run into a lack of "even-ness" between their output levels. That is, unless they have the same gain or amplify signals at the same rate.

Think of having two cars with equal horsepower & torque, but with different gearing. One may deliver all it can much faster whereas the other may take longer to deliver the same amount of power because it can't "rev" as quickly. Using identical motors with identical gearing makes things far simpler and easier to achieve identical results. Sean
>

PS... I was a "gear-head" in a former life, so i can speak that language too : )
Log in to respond
More to discover
80x80
VTL TL5.5 II
- Vacuum Tube Preamplifier
80x80
JADIS ELECTRONICS JP500 MONOBLOCK PREAMPLIFIER
80x80
Marantz 8805A