New Output Transistors for a Quad 405 (2)

[askbojesen]

Well I have great succes by opgrading the output transistors in my Quad 303

Has anybody tried new output transistors for a Quad 405 or a Quad 405-2 ??

I'm curious because I heard that the current dumping technology which 405 uses, makes it critical how fast the output transistors are

Best Regards, Ask

[nickgreek]

Here is the QUAD 504.2 UPGRADE
http://www.net-audio.co.uk/quad405caps.html

[askbojesen]

Yes I know how to change opamps, capasitors and resistors on my 405,2 which I've done with succes

But how about the output transistors??

I'm thinking of what I read in:
http://www.dc-daylight.ltd.uk/Valve-...-405-Mods.html
http://www.dc-daylight.ltd.uk/Valve-...5/405_Qw_6.pdf

405_Qw_6.pdf page 8 quote:

b) The following modifications are without any(!) benefit as long as speakerimpedance doesn t drop significantly below 4 Ohm (and even in that case the audibility is debatable, of course). Usually the 405 is not recommended for this kind of low-impedance-loads -- but have a look ... I did this mod for curiosity-reasons only, and it works nicely indeed. But it requires some 'hard work' and is a pleasure only to her (or him) who enjoys opening the toolbox (purists should skip the rest of this section to avoid heart-attack!).

To get more current with less distortion, you can upgrade each single outputtransistor (17556, 2SD424 -- or even the veteran BDY77) by a pair (yes: a pair!) of up-to-date-devices. Doubling the devices will give safer and better performance with low-impedance loads because each device will work at half the current (where their current-gain is higher) and resistive losses are reduced as well. Thanks to the uncritical class-C design of the dumper-stage (no quiescent current) this upgrade is no problem electrically (as the 606-family shows). Mechanically it has become rather easy thanks to the new TO-3P(L)/TO-264 'plastic'-packages for powersemiconductors.

A state-of-the-art choice for upgrading might be Toshiba's recent 2SC5200 (or 2SC5359) which replaced the recommended 2SC3281 in ~1997 (be careful: by now most devices offered as Toshiba 2SC3281 are just fakes - something like 2N3055s in TO-264-cases!). Motorola's improved copy, MJL3281A, seems to be still in production, and recently ON-Semiconductors introduced the MJL4281A. They all have nearly constant dc-current-gain of about 100 from 10mA up to ~7A (with the older types gain drops from about 50 at 3A to less than 30). But unfortunately they are very fast (CGBP ~30Mhz) and thus not wholly uncritical. There is no benefit from increased dumper-speed here, on the contrary: if the dumpers open too fast, the class-A stage may be too slow (due to C8) to react in time. Test for overshot with 1kHz square-wave. Usually ~1nF (ceramic) from collector to base of Tr10 (like C19 in some issues of the 405-1) will help already. (If you are lucky, C19 and R41/L3 are present on your board [sn. 9000 to 59000]; this will put you onto the safe-side anyway).

Motorola's MJL21194 and 21196 (a kind of improved 15024) are more conservative alternatives: They are not that fast (CGBP ~7MHz) and they show nice current-gain characteristics up to 5A as well (which is, obviously, more than ample, at least with double-output-devices). 'MJ' indicates TO-3 at Motorola, 'MJL' is TO-3P(L)/TO-264, so look for the 'L' here, since all are available in 'classical' TO-3 as well. Maybe even some TO264-versions of MJ15003 are (or will be) available.

unquote

Best regards, Ask

[405man]

I have done a large number of 405/405-2 and I use MJ15003s. I also replace all the electrolytics with bipolar ones and the TL071 with an OPA134. This provides a big improvment to this much maligned amp.

[Zombie]

405man: what bipolars did you use?
Cheers
Tom

[405man]

I use MJ15003s for the output and stick with the 40872s

[K-amps]

Another device used is the used to be popular 2sd424/2sb554 pairs in TO-3.

K-

[askbojesen]

Hi!

And thanks for keeping this Quad 405 thread alive
At:
http://www.quadesl.org/Amplifiers/body_amplifiers.html
I read:
Quote
MAC MOD 405 USA
...
- Current Limiting removed
- Improved IC fitted (LM 318)
- Faster Drivers ( Motorola MJE 15031 )
- Faster Output Devices ( Motorola 2N3773 )
- Improved Resistors (metal-film 1% by Resista Mk3 or Corning RN-60 )
- Improved Capacitors (Wonder Caps, Wima, dipped mica)
- Improved Wiring (Audioquest) and Plugs (Tiffany)
UnQuote

I just wondered if anybody tried something even faster?

Regards, Ask

[405man]

I have used 2N5038s in place of the MJ16003s in the past and they have an Ft of 60Mhz. I could not tell any difference. I was wondering if anyone had tried altering the value of the 560r bootstrap resistors to increase the contribution of the class A amp. When using MJ15003s I also increase the capacitors that determine time constant for the current limit to 100uF makes a small improvment and so far I have had no problems.

[Chris Daly]

I agree MJ15003's are a good transistor for the 405


The 405 has been very popular. Its low distortion is
remarkable. This asset endears the amp to many
listeners. Very few amps manage to get this low
in distortion

On the interesting side

the use of capacitor inductor and resistors in the bridge
circuit has invited lots of discussion asking whether these
components combine properly. The distortion figures
and actual use suggest they are very well chosen for
consumer use. Other values however have been tested
to see if distortion can be lowered further,but not necessarily suiting consumer use.


Cheers / Chris