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Old 30th December 2007, 06:00 PM   #11
hahfran is offline hahfran  Germany
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Join Date: Dec 2007
Default Schematic

The amp which features two design characteristics of the Quad 303 , the triple cascade topology, and inverting, is now designed.
A particular restriction is that the two monoblocks are supposed to sit right behind bookshelf speakers thus the mechanical dimensions are restricted, and thus the size of the heat sinks and their thermal resistance. The loudspeaker features a min. impedance of 4 Ohms at 200/sec and that is just the frequency where the maximal volume appears in most classical pieces of music . Hence the supply voltage had to be reduced and fortunately ebay auctioned 4 toroidal transformers with 25 V and 50 VA each. ebay also auctioned a bag full of Siemens 15000 F/50 Volt and 6800F/63 Volts electrolytics thus this case was solved.
The power transistors SC2922 SA 1216 are directly, without insulation, mounted on the heatsink and so are the 1 Ohms resistors. This spares about 0.3C/Watts thermal resistance
and keeps the PCB off high current.
Transistors T2,T8, T11 are mounted on a 4 by 1 cm copper
between two thermally isolating polystyrene layers and are kept at a constant temperature of 70C by an additional power transistor and a thermostat circuit. Since the temperature of the junctions of these transistors determine the quiescent current
- it is independent of the junction temp. of the power transistors-
the conditions are stable.
The CMRR of a current feedback inverting amp is poor thus the
driver must be fed from regulated supplies which have to put out just 25 mA each.
The input impedance is 1 kOhms thus a preamp is required which can drive this low impedance. Such preamp is already designed.

Transistors: BC449/ BC 450 are Motorola with 100 V BVCE and fT 150 Mhz and hfe 80 min. BD389/ BD 390 are also Motorola which are a bit more rugged than the faster SA 1112 SC 2592 and feature a lower Vsat.

The objective, or rather, subjective is to have an amp which has a feeling for music. It is known that the absolute value of THD is not a measure for the audible quality.
It is rather so that the respective energy of harmonics must
decrease in a particular mathematical series and push pull triode amps for whatever reason produce just such a distribution of harmonics, where the THD is 10% but subjectively they sound better than an amp with THD 0.5%
Happy new year!
Dieter F.
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Old 30th December 2007, 08:40 PM   #12
hahfran is offline hahfran  Germany
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Join Date: Dec 2007
I forgot to attach the schematics. Here it is.
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Old 31st December 2007, 10:18 AM   #13
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Location: HKSAR
Sorry John, forget to ask :

You think the 2n5415 / 2n3440 are qualified too?

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Old 1st January 2008, 05:21 PM   #14
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HI Bigpanda

The difficulties with 2N3440/2N5415 I think is that they have some frequency limitation at Vce < 10V. They also have a gain roll-off after about 100 mA (if I recall correctly) and this can affect drive. They are only about 30 MHz anyway, so I'd prefer the higher frequency BD139/140 drivers. These have a gain hold-up to at least 200 mA.

I've used these for the first of the triple stages, as well as the second. One reason is that if you use these the stabiliser can be a BD139 as well and they all fit onto a metal angle panel to keep them at the same temp.

I'll post a photo of my layout later.

Here's the diagram for my Mod Quad. Some points about this: the faster output transistors (epi 2N3055's) allow double the bandwidth of the original. Simulations show that the original has a BW of about 70 kHz. This one has a BW about 150 kHz.

Overall, the simulated distortion at 20 kHz is slightly higher: comparing simulations, the Quad original has about 0.05% H2, but about 0.07 H3. The Mod has about 0.1% H2 but .05% H3. And this is the point - crossover distortion components are monotonically reducing, so appears to be improved.

The 10 nF roll-off capacitor across the base of the second stage transistors has been removed. Since the first three transistors are inverting, the Miller capacitor can be placed on the input stage instead of the third stage. This prevents the first stage overloading even on a square-wave transient. The new Miller capacitor uses 220 pF. So, now the original 100 pF Miller capacitor can be unwired from the third stage transistor base, and instead is grounded to act as a "sprog stopper" for the output stage.

I've kept diodes as the protection scheme, but other ideas may be better. The upper output stage triplet uses a compensating diode, to make this more complementary-like. This also reduces the saturation voltage of the upper output stage as well, as the original 10 ohms has quite a high voltage drop at high currents. LIkewise, I've removed the lower 10 ohms for the same reason.

I've added a small resistor and rather bigger capacitor in the power supply line mainly to filter the line from R11 which directly introduces supply noise into the amp. It is possible to run the amp. from an unreg. supply (if wanted) but if so, I recommend large capacitors (10mF) and a fairly heavy currenttransformer (I tested my Mod on a 50V 6A unit giving 72V quiescent.) so that there is not much voltage droop between quiescent and full load. I'm sure a stab. PSU will be better if anyone prefers to add one.

Anyway, this circuit is a suggestion - anyone is welcome to comment!

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Old 1st January 2008, 08:44 PM   #15
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Hi all

Picture of proto Mod amp!

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Old 1st January 2008, 09:35 PM   #16
hahfran is offline hahfran  Germany
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Join Date: Dec 2007
as a very simple trick if you have a sine wave generator ( or a program for a sound card ) and a two channel scope measure the difference of the input - the wave generator - and the signal at the base of Tr5 or Tr6 to determine the correct quiescent current
and to see the influence of the 10 Ohms resistor.

Dieter F.
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Old 1st January 2008, 09:53 PM   #17
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Hi Dieter

One trick I have used to use to set the quiescent current of the (original) Q303 was to put the oscilloscope probe onto the VAS transistor. This is inside the feedback loop, and has much higher distortion than the output.

At low settings, and expanded origin showed distinct slope changes, for a sinewave input, around the crossover region. At low bias, the slope increased - and at higher bias, the slope changes to being shallower than the original sinewave. This illustrates the difference between crossover distortion and gm doubling.

(Incidentally, this is one reason to use a current source instead of bootstrap. The bootstrap resistor attached to the base of the upper stage always forces a resistive loading on the two bases. No setting can completely equate the two (exponential) inputs, but a constant current source cannot increase gm. Except, of course, the local Miller feedback or other capacitors reduce the impedance at high frequencies, but by then one hopes that the distortion components are inaudible.)

I found (again on the original) that you could set the bias like this at 1 kHz - but then it was slightly different at 20 kHz. I set the Iq to give lowest visible distortion at 20 kHz. I presume this was due to using "slow" 2N2055's.

I have not tried this on the Mod, I have to say, but I doubt that it will have the same crossover problems as the original. I found that symmetry could be improved either by using a diode (as shown) or by simply re-allocating the lower 1kohm resistor between the base and emitter of the lower NPN driver, instead of to ground.

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Old 1st January 2008, 10:55 PM   #18
hahfran is offline hahfran  Germany
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Join Date: Dec 2007
Hi John
yes I had some good reasons to change the driver completely.
First I have lead lag compensation almost independent of each other and next no miller capacitors at all. However, with a software for a soundcard I am not able to measure distortion - such that the measure is repeatable - except varying noise floor.
This may be due to the very fast Sanken power transistors or whatever I am not an expert in taking exact measures.
Although in theory to put an emitter follower between the base grounded Vas and the complimentary triple cascades in practice
it yielded only problems as the amplification of the Vas became too high and I had to decrease the bandwidth significantly. This may be an effect of improper PCB layout. Nevertheless so far I am
very satisfied with the sound after all subjectively its better than
that of a high grade Mosfet amp. Emotionally better all else is
insignificant because I cannot hear numbers as accurate they may appear.
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Old 8th January 2008, 05:39 PM   #19
hahfran is offline hahfran  Germany
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Default Heat source

Apparently a heat source is the opposite of a heat sink
As the amp has to have a standby power supply I put transistors T2,T11,T8,T4,T7,T5, and T6 in a kind of oven heated by a small power transistor to 75C . The accuracy is roughly 2C . As the devices are always kept constant at this temperature as the heater is fed by the standby power supply it is not a green design but it is very stable.
The "oven" is simply a copper tin strip thermally insulated on either side with 10 mm polysterene layers.
As the sum of power dissipation of all transistors is around 1 watt
the heating must be 3 watts. A next step could be a Peltier element to compensate the temperature increase by power transistor heatsinks .

Dieter F.
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Old 1st March 2010, 01:47 PM   #20
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Location: D-55629 Schwarzerden
Only truly experienced (and unfortunately very few) developers (like Quad) achieve a design of such triple stages without any oscillation under any load condition.

I recall, that I have for checking a diy amp device, that uses also such - slightly different topology:
The name was "Equa" from German magazine "Elektor" December 1972, page 1216
This amp was an unwanted oscillator (even by p-Spice simulation and in real life only to test by current limited power supply without destroy the output devices).
Only after remove one stage in the output buffers I get satisfy operating.
About this address perhaps there is a complete Equa schematic available:
Audioclub Oost Brabant, clubavonden

I want to have basic articles about such three stage super quasi complementary stages. Who knows more about this?

I know too few English keywords therefore. I recall, that I have read some years ago a JAES article. But I don't know if it was the main topic.

Last edited by tiefbassuebertr; 1st March 2010 at 01:52 PM.
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