No idea at the moment mate, sorry, I don't have time to take one apart to measure.
But at a guess I'd say about 1mm.
As I said, L3 & L4 are not a core part of the circuit.
They're to 'Slow down' the output of the 'Current Dumpers' to avoid misbehaviour at HF.
Quad put L3//R41 between the top dumper and +ve rail on some boards, but not others.
But at a guess I'd say about 1mm.
As I said, L3 & L4 are not a core part of the circuit.
They're to 'Slow down' the output of the 'Current Dumpers' to avoid misbehaviour at HF.
Quad put L3//R41 between the top dumper and +ve rail on some boards, but not others.
Thanks John, its handy to know they can be shorted. I recall seeing a site with 22uH in the two positions, for why though?
Ah, got it. I replied before seeing this post John. Oh my question regarding why the 22uH rather than the 6.8uH still stands. I wish I could remember the source, it was part of several modifications, I think.
Yes, the 'other' position is L3 between the top Dumper & + rail.
I guess Quad were experimenting with the best layouts and values on different boards.
But the fact that they changed it so much shows the exact value is not critical.
They changed it to 4uH on the 909 model - a much later version of the same basic circuit.
They still use the basic Current Dumping circuit in their new 'Elite' amps,
which sell for $1600 US.
There' a review of it here by Ken Kessler
I guess Quad were experimenting with the best layouts and values on different boards.
But the fact that they changed it so much shows the exact value is not critical.
They changed it to 4uH on the 909 model - a much later version of the same basic circuit.
They still use the basic Current Dumping circuit in their new 'Elite' amps,
which sell for $1600 US.
There' a review of it here by Ken Kessler
I was looking at the Quad website last night and realised they still have an incarnation of the current dumping design. Mouser have what I need, I am not used to shelling out money for parts, I used to give a friend beer tokens and he would add my modest list onto his monthly stock order.
I found a thread on here relating to capacitor quality and a question as to which caps should be polystyrene. The question was not answered but I make a rash assumption to use quality units in the signal path?
I found a thread on here relating to capacitor quality and a question as to which caps should be polystyrene. The question was not answered but I make a rash assumption to use quality units in the signal path?
Here's what I did with the caps I changed.
I went for good quality but not 'magic audiophile' -
I went with Silver Mica for C6,7,8 & 11
C1 & C4 should be the same type to try to have equal and opposite distortions. I went with Panasonic ECW-FE2W just because Mouser had those values in a good quality cap at the time. I got a bag of Kemet R75 for decoupling of supplies etc.
Electrolytics in the signal path, I went with Silmics or Nichicon Muse.
The only Interesting/different thing I did with the caps was to use Bipolars for the lower value Electrolytics in the signal path. This is traditionally viewed as bad by audiophiles.
But I was guided into this by a series of studies done by Cyril Bateman in Electronics World 2002-3. He actually measured the distortions and found Bipolar to be lower.
I went for good quality but not 'magic audiophile' -
I went with Silver Mica for C6,7,8 & 11
C1 & C4 should be the same type to try to have equal and opposite distortions. I went with Panasonic ECW-FE2W just because Mouser had those values in a good quality cap at the time. I got a bag of Kemet R75 for decoupling of supplies etc.
Electrolytics in the signal path, I went with Silmics or Nichicon Muse.
The only Interesting/different thing I did with the caps was to use Bipolars for the lower value Electrolytics in the signal path. This is traditionally viewed as bad by audiophiles.
But I was guided into this by a series of studies done by Cyril Bateman in Electronics World 2002-3. He actually measured the distortions and found Bipolar to be lower.
I’m upgrading a recently acquired 405-1 (ser.no. 2098, quad service in 1993) based on Bernard Ludwig’s recommendations, which are briefly:
Outputs to MJ 15003
D1/2 to 15v
IC1 to NE 5534, D1/D2 to 15v, R6 to 100k, R4 to7k, 75ohm across L2, R7/8 3.3k uprated.
C2 add 100nf, C4 to 1.5nf, c4/5/13/14 replace, C11 to 1000pf, 100nf to IC supply pins.
C15/16 add 100uf, fit mains filter cap, fit 470nf to main TX outputs.
It has a crowbar board attached to speaker sockets, this will have new Triacs and moved further inboard.
It came with the sensing resistors having already been bridged with short “sensing” links/wire, so not sure about these.
He also recommends that the cabling is re-routed which will better isolate the signal line.
When finished and tested, the amp is to drive ELS63/ 57s. I am aware of the R11 link. I know that there are many other mods, but the aforementioned is within my understanding and ability, but if there are any other ones please suggest.
Outputs to MJ 15003
D1/2 to 15v
IC1 to NE 5534, D1/D2 to 15v, R6 to 100k, R4 to7k, 75ohm across L2, R7/8 3.3k uprated.
C2 add 100nf, C4 to 1.5nf, c4/5/13/14 replace, C11 to 1000pf, 100nf to IC supply pins.
C15/16 add 100uf, fit mains filter cap, fit 470nf to main TX outputs.
It has a crowbar board attached to speaker sockets, this will have new Triacs and moved further inboard.
It came with the sensing resistors having already been bridged with short “sensing” links/wire, so not sure about these.
He also recommends that the cabling is re-routed which will better isolate the signal line.
When finished and tested, the amp is to drive ELS63/ 57s. I am aware of the R11 link. I know that there are many other mods, but the aforementioned is within my understanding and ability, but if there are any other ones please suggest.
Hi, You have a really early amp there.
It looks like it might have blown the OP transistors at some stage.
R38, R31, C5, Op Amp, Fuses, all have been changed.
The wire sensing resistors might be original? they're only about 0.08ohm so can be made of resistance wire.
I definitely thought that deleting the current limiters made an improvement - you won't need them anyway with the new O/P trannies.
This was also the biggest source of complaints made about the original amp.
The 405 family were the first of one of the most successful high quality amplifiers.
The current dumping circuitry has been used in over 200,000 Quad amplifiers, from 405 and 405-2 through 306, 606, 707, 909, the 510 series pro audio amps up to the present day Elite series QSP& QMP amps.
Good luck with it!
It looks like it might have blown the OP transistors at some stage.
R38, R31, C5, Op Amp, Fuses, all have been changed.
The wire sensing resistors might be original? they're only about 0.08ohm so can be made of resistance wire.
I definitely thought that deleting the current limiters made an improvement - you won't need them anyway with the new O/P trannies.
This was also the biggest source of complaints made about the original amp.
The 405 family were the first of one of the most successful high quality amplifiers.
The current dumping circuitry has been used in over 200,000 Quad amplifiers, from 405 and 405-2 through 306, 606, 707, 909, the 510 series pro audio amps up to the present day Elite series QSP& QMP amps.
Good luck with it!
Thank-you for your reply.
I had already began changing some components before I took the photo.
Most of the mods by Bernd Ludwig have now been carried out.
One sensing resistor R36 has already been shorted (when I bought the amp) with a short piece of resistance wire on the print side?
Which current-limiting components need deleting?
Thank-you.
I had already began changing some components before I took the photo.
Most of the mods by Bernd Ludwig have now been carried out.
One sensing resistor R36 has already been shorted (when I bought the amp) with a short piece of resistance wire on the print side?
Which current-limiting components need deleting?
Thank-you.
If R35 & 36 are shorted on the print side, that will stop the current limiters activating.
You can then cut off and remove R 24, 25, 26, 27 ,28, 29; D3 & 4 and TR 5 & 6
Which are all the components of the current limiting circuitry.
You can just snip them off the components side - no need to unsolder.
C3 - the compensation cap for the old op amp - may well not be needed with more modern designs, depending on the specific op amp chosen.
You can then cut off and remove R 24, 25, 26, 27 ,28, 29; D3 & 4 and TR 5 & 6
Which are all the components of the current limiting circuitry.
You can just snip them off the components side - no need to unsolder.
C3 - the compensation cap for the old op amp - may well not be needed with more modern designs, depending on the specific op amp chosen.
Your diagram "my mods" in post 1- should there be a connection from the output of r20/21 to junction of tr9/10?
Still can't get one of my boards working, everything checked out, matching resistance checks on both boards, new Mj 15003s and many other components, but my safety 60w bulb is half lit, high frequencies curtailed and bass distorted (using cheap test speakers).
All sorted now, found that a pair of output transistors were leaky.
Only thing that concerns me is the output bias voltages (which I read should be below 0.5v.) are puzzling.
Prior to fitting modification D13/10 ohm res, bias volts on Tr9=0.57, Tr10= 0.4.
after fitting D13/10ohm, volts on Tr9=0.16 Tr10= 0.47.
Only thing that concerns me is the output bias voltages (which I read should be below 0.5v.) are puzzling.
Prior to fitting modification D13/10 ohm res, bias volts on Tr9=0.57, Tr10= 0.4.
after fitting D13/10ohm, volts on Tr9=0.16 Tr10= 0.47.
That is the whole intent of the mod. It is to subtract a diode drop from the Vbe of Tr9. Check the circuit.