Quad current dumping amplifiers

[Radfordman]

I use a Quad 405 in my PA system. Much nicer than some of the PA amps. ( Stageline, for instance) When I play music through it people comment on the quality. I've never actually tried it on my own speakers, LS3/5as. I'm told it is a good match. Martyn .

With care with LS3/5a, I would say, preferably with power limiters in.

In 1976 i bought a new 405. My speakers were home built based on Martin Colloms Mini Monitor design. KEF B200 SP1014, Celestion HF1300 and Coles 4001G. The cabinets I purposely made bigger than Colloms design. They were (still are) pretty good.

However the B200 SP1014 does not last very long with fed with that sort of power. Very quickly I blew one B200. Sent both B200's back to KEF for repair. They did a nice job, but I always used the 405 with the power limiter on after that.

 

[Robert]

My first 303 did just that back in the early 80s, it spewed one of it's caps all over the circuit board and a channel died. I think that was an old amp, almost certainly a Mk I. I bought it second hand in 1978. The one I have now also has the caps in with connections downwards, though it has never leaked. When I had it serviced Quad replaced all four with ones that are a lot smaller physically, but they didn't changed the orientation. Looks like they have used 2200uf caps, at least I can read that on one of them through the vents, I've not popped the lid off for a proper look. I like this 303, it's in immaculate unmarked condition, in fact far better than the one I bought 32 years ago which had a slight ding on the top!

Tony.

Worth bearing in mind that the value of those caps in the original 303 will have almost certainly been determined by what was available at the time. Large capacity high voltage caps were physically very large and expensive.

The speaker coupling caps should theoretically be as large as possible, smaller ones will simply roll-off bass as load impedance falls and kill any electrical damping. Not much of an issue with 16 ohm loads but certainly a factor with lower impedance loads.

 

[Tony L]

Uprating the coupling cap value seems to be a very popular tweak for the 303. I've deliberately not gone down that path as I wanted to reacquaint myself with the stock amp, and beyond that laziness kicked in - it's working fine into the spectacularly easy to drive Heresys, so I'll leave it alone!

Tony.

 

[Robert]

If anything, you are possibly getting the benefit of two negative making a positive there.
Plus the Heresy is well damped mechanically so doesn't need or want amplifier damping.

It drives 57s well for similar reasons.

 

[Analog addict]

I use a Quad 405 in my PA system. Much nicer than some of the PA amps. ( Stageline, for instance) When I play music through it people comment on the quality. I've never actually tried it on my own speakers, LS3/5as. I'm told it is a good match. Martyn .

Me too, I run two 405s with another one for back up. They drive my two pairs of 15" McGregors loaded with Eminence drivers really well.

 

[Tony L]

If anything, you are possibly getting the benefit of two negative making a positive there.
Plus the Heresy is well damped mechanically so doesn't need or want amplifier damping.

It drives 57s well for similar reasons.

The Heresys are very high impedance, they range between about 10 Ohms and 72 Ohms according to a plot in one of Paul Klipsch's 'Dope From Hope' newsletters (p64 of the .pdf file). He makes a case that this is a good thing, though it goes over my head as I'm not electronics literate. I have a lot of respect for PWK though, he talked a hell of a lot of sense.

Tony.

 

[Robert]

Yours sound great Tony, and not at all as I'd have expected.

I think high impedance loading combined with high sensitivity does give the amplifier a very easy ride so perhaps that is what PKW means. It effectively means that any clean amplifier should sound great.

I'm working on plans at the moment for a BBC style box, slightly taller and deeper than a BC1 using a similar driver line-up but with a true 8 ohm load, 90dbw sensitivity and decent bass extension with aperiodic loading.

 

[Tony L]

I'm working on plans at the moment for a BBC style box, slightly taller and deeper than a BC1 using a similar driver line-up but with a true 8 ohm load, 90dbw sensitivity and decent bass extension with aperiodic loading.

Look forward to hearing them. Are you going 'thin wall'?

Tony.

 

[Robert]

Yes the works.

Ply for the cabinet, screwed front and back.
The format will follow the BC1/SP1/LS3-6, so bass/mid, large main tweeter and smaller upper tweeter to improve power response at the top.

There are some really good doped paper 8" bass drivers around these days that offer good sensitivity and go low, given a nice big box.

 

[Tony L]

Excellent, that sounds like a proper speaker recipe to me!

Tony.

 

[Radfordman]

If anything, you are possibly getting the benefit of two negative making a positive there.
Plus the Heresy is well damped mechanically so doesn't need or want amplifier damping.

It drives 57s well for similar reasons.

I read a technical article somewhere that talked about the interaction of the ESL57 with the 303 (with standard output capacitors). So for the 57 the standard 2000/ 2200 uF may be best.

This looks like it might be the article:

http://www.audiomisc.co.uk/57and303/interact.html

However in to some moving coil speakers, I felt an improvement with increased value output capacitors.

 

[Radfordman]

I read a technical article somewhere that talked about the interaction of the ESL57 with the 303 (with standard output capacitors). So for the 57 the standard 2000/ 2200 uF may be best.

This looks like it might be the article:

http://www.audiomisc.co.uk/57and303/interact.html

However in to some moving coil speakers, I felt an improvement with increased value output capacitors.

I am surprised that with all the interest in ESL57's, 303's and other amps to drive the 57's that no one has commented on the above article, which to me was very interesting.

 

[Tony L]

I am surprised that with all the interest in ESL57's, 303's and other amps to drive the 57's that no one has commented on the above article, which to me was very interesting.

I'd seen it before, but it's a little out of my understanding as, to be honest, I don't really know what impedance is! Anyway, here's a plot for a Heresy...

I suspect this makes my 303 is a pretty happy one, but again I don't have the fancy math to prove / disprove this.

Tony.

 

[Radfordman]

I'd seen it before, but it's a little out of my understanding as, to be honest, I don't really know what impedance is! Anyway, here's a plot for a Heresy...

I suspect this makes my 303 is a pretty happy one, but again I don't have the fancy math to prove / disprove this.

Tony.

Well, I suppose being a qualified mechanical and production engineer and spending most of my working life working in electronics (I still find that strange) and having had an interest in Hi Fi for over 40 years, I stupidly expect others to have the same viewpoint as me.

Anyway, 303 + ESL57 = a pretty happy marriage.

And that impedance plot of yours look as those those may be an easy load for the 303, as mentioned the 303 has lower distortion into higher impedances.

 

[Robert]

I'd seen it before, but it's a little out of my understanding as, to be honest, I don't really know what impedance is!
Tony.

Tony, the easiest way to describe it would probably be resistance related to frequency.

Pure resistance as in 'this length of wire has a resistance of 2 ohms', is only applicable at DC. Move to AC and the resistance of the wire will alter depending on the frequency of the AC signal, because the characteristics of the wire and the load at each end mean that the wire cannot transfer all frequencies equally.
Simplifying wildly but hope you get the gist.

 

[Robert]

I am surprised that with all the interest in ESL57's, 303's and other amps to drive the 57's that no one has commented on the above article, which to me was very interesting.

I think it was mentioned here previously, but it might have been somewhere else.

Essentially, a 303 has a very mild 'loudness' contour effect on the ESL. It very subtly lifts low bass and high treble.

But the ESL doesn't stress 303 at all - where music related power demands are greatest the ESL presents a high impedance load.

 

[Radfordman]

Tony, the easiest way to describe it would probably be resistance related to frequency.

Pure resistance as in 'this length of wire has a resistance of 2 ohms', is only applicable at DC. Move to AC and the resistance of the wire will alter depending on the frequency of the AC signal, because the characteristics of the wire and the load at each end mean that the wire cannot transfer all frequencies equally.
Simplifying wildly but hope you get the gist.

If this piece of wire is in the form of a coil, yes above is true, but if it is just a straight piece of wire (we will assume zero inductance) the DC resistance will be the same as its AC 'resistance'.

I'm sure someone can explain better than me.

 

[Radfordman]

I'm using the 303 as it costs very little to run (my back room system is on most of the day)

Tony.

This is one of the reasons I'm now using a Teleton SAQ206B, small amp runs cool, is on all day. It's actually a very good sounding amp. Teleton was made by Mitsubishi. A fact that is not widely known.

 

[Robert]

If this piece of wire is in the form of a coil, yes above is true, but if it is just a straight piece of wire (we will assume zero inductance) the DC resistance will be the same as its AC 'resistance'.

I'm sure someone can explain better than me.

Even straight wire has some effect, albeit up at ultra high frequencies and RF due to things like skin effect, and of course proximity to other conductors if near - and we can assume that in audio.

But I'd agree that at audio frequencies, you can rightfully sub resistance for impedance when talking about most wiring.

 

[Robert]

This is one of the reasons I'm now using a Teleton SAQ206B, small amp runs cool, is on all day. It's actually a very good sounding amp. Teleton was made by Mitsubishi. A fact that is not widely known.

Didn't know that - had an old Teleton as a kid. I remember a wooden sleeve, square silver pushbuttons and sliders
Oh and ceramic as well as magnetic phono inputs.