SpeakerTalk

centaurus · Posted: Thu Sep 28, 2006 12:39 am Post subject: Quad 405 info needed

i'm still a vintage tube guy at heart, but i know my 104/2 will sound better with a 405 than my mcintosh MC60's.

so, some quick questions. being these are sorta old, do they sound really "transistory" in the high frequencies?

what makes them better than other solid state amps from the same era?

how much better are the series II versions compared to the original 405?

Robby

speakerguru · Posted: Thu Sep 28, 2006 12:07 pm Post subject:

The first 405 had a straight voltage-dependant-current-limiter. Once you reached the threshold it would reduce the output current as the output voltage increased. This is the behaviour of a negative resistance. If the resistive part of the loudspeaker impedance was below the value of the negative resistance of the 405 in limiting mode what you ended up with was an oscillator. So, the amp would not only clip, it would bounce between the power supply rails. Very nasty sounding. This used to happen with the old pre-aB 104, which used to drop down to under 3 Ohms around 3 kHz. There were also problems with the Cantata.

KEF brought out the 104aB which had a much "nicer" impedance and Quad modified the 405 limiters to have some sort of averaging rather than instant operation in the series 2.

Model 104|2 has close to a pure resistive 4 Ohm input impedance and so there will be no problems with 405 limiter operation. The small heatsink, however, may not be adequate for long term use at full output.
SG

centaurus · Posted: Thu Sep 28, 2006 5:16 pm Post subject:

Thanks!

i'm casually looking at some macmod 405 amps (mccormack modded monoblocks) i'm a tube guy at heart, but a quad KEF experience would be fun!

see ya,
Robby

ColinR · Posted: Thu Sep 28, 2006 8:11 pm Post subject:

speakerguru · Posted: Fri Sep 29, 2006 11:55 am Post subject:

Sorry Colin I seem to often end up disagreeing with you. Bridge mode does not make a 405 more suitable for driving low loads (except for heatsink sharing). I will explain:

If you invert the input to one of the amps and use both in bridge mode to drive one speaker you are potentially doubling the VOLTAGE output. The CURRENT limit stays the same (unless you mod the current limiters).

It works like this: if you apply +2 Volts to one end of a 4 Ohm load and -2V to the other end of the load you will get 1A in the load. So each amp is putting out 2V and 1A so they are each "seeing" a 2 Ohm load.

If you want to double the current you have to put the amps in parallel, which they won't like at all unless they are gain matched to perfection including tracking at all outputs and temperatures. A no-no.

In general if an amp is designed for R Ohms you can use it in bridge mode with a 2R load without any current limit problems.

SG

ColinR · Posted: Fri Sep 29, 2006 2:37 pm Post subject:

Suitably chastised I'll dig a slightly deeper hole Smile

.

If you look at the current/voltage graphs supplied in the manual by Quad for the stock 405 & 405-2, they are driven into current and voltage limiting at lower RMS output levels with 4 Ohm loads.

Quad produced the parallel bridging kit to obviate this effect for speakers like the Ref 104-2 due to too many failures.
The existing power amplifier modules should be "matched quartets" btw before modification.

A stock 405-2 driving a 4 Ohm resistive load sounds rather "edgy" to my ears (clipping of some kind?).

Having always taken Quad's word at face value due to the novel nature of the design I just trotted out the Quad mantra, I've got all of the Quad paperwork and can photograph the bridging modules if you'd like some copies.

A series modification for 15 Ohm loads was also produced using an input transformer.

Quad's bridging instructions can be found here

http://quad405.com/405monorev1.pdf

plus several other modifications at the parent site http://quad405.com/ .

The only worthwhile one btw is changing the capacitor in the input signal path to a Black Gate - they weren't available when the 405 was designed, but probably would have been included if they had been Shocked

.

My Quad brood consists of 4 405-2's, 2 4010-2's, 3 606-2s, 520f, 520M, etc.

Finally for those not in the know a F, M or C is a B.B.C. unit; f, m & c are "broadcast standard" and the 606-2 never existed as an official model Cool

.
_________________
This post or any other information supplied to this website or any other by myself is not available for any form of commercial purpose i.e. to hi-fi magazines or as sales and marketing material for sleezeBay or Audiodogging pimps and the like.

centaurus · Posted: Fri Sep 29, 2006 5:41 pm Post subject:

i've heard great things about the macmod 405's. many say that the monoblocks sound lightyears beyond stock models. they suck?

i'm just casually looking at this point, but i was offering my Conrad Johnson PV1 tube preamp factory modded to PV2a specs for trade or partial trade.

Robby

ColinR · Posted: Fri Sep 29, 2006 8:47 pm Post subject:

centaurus · Posted: Thu Oct 12, 2006 5:32 am Post subject:

well, i am potentially buying a minty 405-2 and 34 preamp with orginal boxes and manuals. made in 1984 and from the original owner. $250 USD for both with all the DIN connectors and banana plugs.

there's some saying that the 405-2 is overmatched by the 104/2, but i'll give it a whirl. he's coming over tomorrow and he said if it doesnt sound good, i don't have to buy it.

so, here's the question, can this thing, with the necessary mods (new caps, op-amps and removal of the current limiter) rock the 104/2? or am i wasting my time and money (although i'm sure i can sell them to at least get my money back out of them).

the combo is just so cute looking Laughing

Robby

ColinR · Posted: Thu Oct 12, 2006 6:46 pm Post subject:

centaurus · Posted: Thu Oct 12, 2006 7:55 pm Post subject:

thanks Colin!

i think even if they don't have enough current for the 104/2 they will:

A. still have more current than my mcintosh MC60 tube amps
B. even if i sell the 104/2 (too picky with amps), i bet the 405-2 would sound mighty nice with my KEF Calindas.

what do you think?

Robby

ColinR · Posted: Thu Oct 12, 2006 10:32 pm Post subject:

centaurus · Posted: Thu Oct 12, 2006 10:57 pm Post subject:

thanks Colin,

i will try them with the calindas too. in your opinion, should i keep farting around with the 104/2 or are they enough of a hassle that i should just sell them and enjoy the calindas with my REL Q100 sub?

the 104/2 look real cool, but if they need uber wattage, like Krell, Naim, musical fidelity ,etc. maybe i should just send them down the road?

blowing $1,500 USD for a suitable amp for speakers i might not keep seems stupid, right?

how would you rate the 104/2 against the 1970's KEFs? those bandpass woofers seem like more trouble than they are worth. and they still don't go below 50Hz. what's the big deal with them?

thanks,
Robby

ColinR · Posted: Thu Oct 12, 2006 11:16 pm Post subject:

centaurus · Posted: Thu Oct 12, 2006 11:26 pm Post subject:

Hi Colin,

i mean i don't need the money or anything. i could just put the 104/2 in the garage for now. of course, the impetus for the quad 405-2 was to power the 104/2. but now it sounds like i might buy the quads because they are cool, have lots of potential and would be an interesting point/counterpoint to my tube mcintosh gear.

have you ever had a pair of 104/2 or spent some time with a pair. what did you think of them and how would you compare them to the calindas. so far, the calindas are a blast! the 104/2 always seem to have the potential to sound better, but so far, they leaving me unsatisfied. of course more power would be key. but i simply don't get the need that much current and power for a speaker with 5 drivers that don't go below fricken 50Hz!

at first, i dug the 104/2, but i mistook the sloppy bass for warmth. the calindas sound much more balanced and trasparent.

your opinions?

thanks,
Robby