Bridging Forte 4a amplifier?

[clm811]

I have a pair of Forte Model 4a stereo power amplifiers. I'm interested in running one stereo unit per side in bridged mode for higher power output. I have not seen this done before, and am curious if there's a reason why not.

This model features "balanced" inputs in addition to the standard single ended inputs. I understand that in bridging an amp, one side requires an inverted signal. I believe the balanced input would provide a means for this...

(How) can one go about (safely) bridging the amplifier using these balanced inputs?

TIA

-Chas

[nigelwright7557]

Originally Posted by clm811

I have a pair of Forte Model 4a stereo power amplifiers. I'm interested in running one stereo unit per side in bridged mode for higher power output. I have not seen this done before, and am curious if there's a reason why not.

This model features "balanced" inputs in addition to the standard single ended inputs. I understand that in bridging an amp, one side requires an inverted signal. I believe the balanced input would provide a means for this...

(How) can one go about (safely) bridging the amplifier using these balanced inputs?

TIA

-Chas

I would guess you use the none inverting input on one amp and the inverting input on the other amp with unused inputs to ground.

[jacco vermeulen]

How many of the GT20D101/201 output devices does the 4a have ?

[clm811]

...use the non-inverting input on one amp and the inverting input on the other amp with unused inputs to ground.

Would someone elaborate on the input/output connection (ref. photo below)?

How many of the GT20D101/201 output devices does the 4a have ?

There are two pairs(4) of each type per amplifier channel. I've heard these amps can successfully drive a 2 ohm load- any comment?

Thanks.

-Chas

[AMV8]

Jacco
The 4a that I have has four devices per rail per channel ( I just checked ). I know it can drive 3 ohm as I have used it in that mode most of its time. I does not get hot driving 3 ohm speakers.
My findings are that it is a detailed and refined sound. I am not sure that it would be my choice of amplifier for high volumes.
Don

[clm811]

I am not sure that it would be my choice of amplifier for high volumes.

BTW my interest in bridging these amps I have on-hand is twofold...

1. My speakers are 90dB sensitive, so I typically average only a few watts. I have many SACD, DVD-A and other Hi Res source materials, hear there are advantages to bridging and I would like to have more than their rated 50Wpc available for unclipped peaks, based upon the tests run at RMAF:

Cordell Audio: RMAF Workshops: Workshop #5 (The Peak Power Demands of Well-recorded Music)

2. I wish to experiment with using a "balanced" high-level passive crossover, which I am creating for my (high resolution) loudspeaker system:

Idea for "Balanced" Passive crossover- opinions?

I hope to test for any audible improvements, primarily in noise reduction, as I live in an area with high RF interference(and have been trying many other noise reduction techniques in my system with varying degrees of success). Moreover, I think its a novel idea, and want to pusue it.
Also, I will employ shielded twin lead speaker cables from +/- outputs to speakers(shield to ground).

My only obstacle at this time is determining how to safely connect the amps in "BTL" (bridge-tied load) mode.

-Chas

[K-amps]

I would not risk the flakey IGBT's with bridging... you they get toasted, so do you.

[jacco vermeulen]

Those early IGBTs by Toshiba are obsolete, replacement cost will likely be in the order of $25 and up each.
Even more serious issue would be the matching, of which Papa stated he already considered a hurdle for serial manufacture.

Later generation IGBTs have square SOA areas, latching of the MOSFET input part cancels secondary breakdown.
First generation IGBTs as the Toshiba complementaries do suffer from second breakdown, see datasheet, making them behave more like BJT power devices.

The Forté audio Model 6 is IGBT powered too, 200W/ch in AB, also runs 4 pairs of the 180W Toshy D101/201 per side.
(in NP style, Forté boards are pretty universal, so i gathered. The 100W Model 5 uses the same boards, with half the output seats unsold)
Bridged mode operation of Model 4 channels results in less peak output current compared to a Model 6, a bridged Model 4 will clip earlier.
On the other hand, thermal load of the bridged channel output devices will be higher due to the elevated quiescent current level, despite the larger fin number heatsinks on the Class A models.
I gathered the higher powered Forté IGBT models didn't turn out as reliable.

Have to confess i bought some of the GT20D101/201 in the early 90s, but chickened out of actually using them for the same reasons.
Fortunately my borderliner ex-spouse kept me occupied enough with plenty other compelling matters at the time.

[AMV8]

clm811
I use the 4a to playback classical music. It handles the wide dyanamic range in classical music well and plays loud passages in the music well.
If you want to play rock music I would choose an amplifier designed to handle high currents. They are different designs of amplifier - such as Krell.
I have never found the IGBD's to be unreliable, but then again I do not drive my amplifiers into high temperatures or clipping.
Don

[clm811]

I do appreciate your responses. It sounds as if some of you have first-hand experience with these amps.

Therefore, I do have more questions...

I would not risk the flakey IGBT's with bridging...

O.K. Not sure what you mean by "Flakey".
Do you have actual experience with these? I have several of the Forte IGBT amps (Models 4, 4a, 5, 6 and FT1) and haven't had a failure under any conditions(yet).

Bridged mode operation of Model 4 channels results in less peak output current compared to a Model 6, a bridged Model 4 will clip earlier.

.
From what you write it sounds like you have tried the 4a in bridged mode? If so, what loads did you place on it and how did it perform? I don't really need 200 watts, but expect the 50 Watt may clip on some peaks.

On the other hand, thermal load of the bridged channel output devices will be higher due to the elevated quiescent current level, despite the larger fin number heatsinks on the Class A models.

May I ask how bridging changes the quiescent current (I thought quiescent current is trim-pot set on the board, and that bridging is just another method of connecting a pair of otherwise unchanged amplifier modules)?

Also, I've counted the fins on both the 4a and a model 6, and they both have identical heat sinks (i.e. twelve 2"x6" fins per side), so which two models were you comparing?

I gathered the higher powered Forté IGBT models didn't turn out as reliable.

I read the Model 6 ran its supply rails much higher(78v) than the model 4, and very close to the limit of SOA of the outputs. This is why I the Model 4/4a with its more conservative 39v rails. Consequently, the 4a's seem to coast along easily on a 3-4 ohm load. I'm hoping that bridging will work acceptably.

Not arguing the facts, just asking for clarification.
Thanks for any advice.


-Chas