Including speaker leads into feedback loop

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08-04-2010, 12:21
tannoydmt

Including speaker leads into feedback loop

I know there was a commercial design for this some years ago by DPA audio amplifiers for their transistor amps, but I thought one day why not try it out on my then Leak stereo 20 valve amplifier as the feedback connection can be very easily taken from the output transformer to extend it out to the speaker terminal thereby including any speaker lead losses within the feedback loop(it is also very easy to reconfigure back to the original if you don't like it). Although I haven't got the leak anymore I thought the results were very good and interesting,although I didn't try any different wiring in the extended feedback wire I suspect this could have an effect and may be worth experimenting with. There is one important pervisso however you must make doubly sure that the extended feedback cable is always connected with a good connection at the amp and speaker ends otherwise the gain of the amplifier will be increased substantially and may result in damage to your equipment.
With modern amps based largely on complicated circuit boards this mod is not easily carried out, but on older hard wired valve amps this can easily be done .
08-04-2010, 12:25
Barry

I have seen this done using the Quad II valve amps. If anyone is interested, I'll try to dig out some information.
08-04-2010, 16:27
zaidstone

In the early 80's Toshiba and Trio/Kenwood offered solid-state amps with this feature. I have a Aurex (Toshiba) SB-77 amp where its called 'Clean-Drive' (I believe its 'Sigma Drive' on the Kenwoods). The additional cable runs from the negative terminal of the speaker (as I remember it, this was to include the speaker xover in the feedback loop) to a extra pair of terminals on the amp.

As I recall the SB-77's manual stated the cable quality for the circuit was unimportant - it was supplied with about 30 foot of near bellwire quality cable to make the connection. The feature is switchable from a front panel control button and having used the Aurex with several different speakers and cables I have to say I have never noticed any difference. Mind you, a very cool looking 'Clean Drive' logo lights up when its on!

I guess any improvement is quite speaker/cable dependant. Or I have cloth ears...
08-04-2010, 18:48
DSJR

Far more important to me is the amp output impedance, as a high one (low damping factor) can have a negative effect on the behaviour of the crossover apparently.
10-04-2010, 13:37
Haselsh1

I used to have one of the Kenwood Sigma drive amps around 1985 but all I remember about the thing was its phenomenal damping factor.
24-04-2010, 17:22
slipperyjim

Trouble with this is that the feedback loop will still try to null out the difference between the input signal and the fed back signal from the speakers which are now at the end of the feedback cable. So, in taking out the effects of the speaker cables, you introduce the effect of the feedback cable. Strongly suspect that either method makes cock all difference. Buy decent gear and all this cable fetishism is for naught anyhow.

More interesting was the Philips Motion Feedback idea which took feedback off a sensor on the speaker cones. It was, unfortunately, implemented by Philips.
05-12-2011, 10:15
The Grand Wazoo

From the Grave
05-12-2011, 16:58
Reid Malenfant

Quote:

Originally Posted by Haselsh1

I used to have one of the Kenwood Sigma drive amps around 1985 but all I remember about the thing was its phenomenal damping factor.

I picked one up from a car boot sale some time in the 90s. Had a glass panel accross the bottom of the amplifier front if I remember correctly? :scratch: I think I sold that amp twice :eyebrows: The first time it was connected up with a small to medium sized pair of Mission bookshelf speakers & it was very interesting indeed. The amp had bass control similar to a class A amp would, or as Shaun mentions the damping factor was pretty high. I ended up getting it back cheap & then sold it to another friend who was incredibly happy with it, until he was burgled & had nearly all his system & records nicked :(

Quote:

Originally Posted by slipperyjim

<snip> Trouble with this is that the feedback loop will still try to null out the difference between the input signal and the fed back signal from the speakers which are now at the end of the feedback cable. So, in taking out the effects of the speaker cables, you introduce the effect of the feedback cable.

More interesting was the Philips Motion Feedback idea which took feedback off a sensor on the speaker cones. It was, unfortunately, implemented by Philips.

Actually the feedback cable from the speakers has vastly less effect on the sound. This is simply because it's carrying hardly any current at all, it's only effectively carrying the feedback voltage. It's the huge current swings on the main loudspeaker cable which cause AC voltage to be dropped accross that cable according to the AC impedance of the speaker that the feedback idea gets around. Effectively the amplifier is directly connected to the speaker terminals, or that's how it appears.

As for the Philips MFBs I tend to agree having fixed one on a previous occasion ;) They certainly aint pretty inside :eyebrows:
05-12-2011, 17:06
Reid Malenfant

Quote:

Originally Posted by tannoydmt

<snip> There is one important pervisso however you must make doubly sure that the extended feedback cable is always connected with a good connection at the amp and speaker ends otherwise the gain of the amplifier will be increased substantially and may result in damage to your equipment.

Actually it can be easily done so it's 100% safe even if the wire gets disconnected ;) The same kind of trick is used in remote sensing on switch mode power supplies that I used to work on.

All you need to do is add something like a 10 - 100 Ohm resistor between the normal feedback point on the amplifiier & the remote sense terminal. With no external connection the feedback is take from the standard amplifier output. With the sense wire in place it will effectively short out that 100 ohm resistor & take the far end of the speaker cable as the feedback point.

Not very handy if there is some form of inductor on the output in case of capacitve loads in the case of a solid state amp though :eyebrows: It also doesn't take into account the ground return from the loudspeaker which is much more difficult to factor in like the Trio Sigma Drive amps..