Tuesday, January 31, 2006

Quadrevision Live: The Quad/Rogers 405 studio monitor

A very interesting amplifier has fallen into my hands!

Quad used to make OEM amps for the BBC, as studio monitors.

Quad worked together with Rogers, which made an active 2-channel filter with symmetric .650 V inputs and the loudspeakers that go with it.

The left channel of the 405 is used for the high tones, the right channel for the bass-tones .

The tweeter-channel is limited with the voltage-limiter originally intended by Quad for connecting the ESL speakers. Nice ;-)

Specific Neutrik-cables were designed for the set to connect the speakers and to connect the inputs to a symmetric pre-amp or mixing-table.

Revision step 1: Making a phase-shifter

As the input is a symmetric one we'll have to make a phase-shifter in order to connect the signal-generator in a symmetric way to the filter for measuring-reasons. With a breadboard and a single transistor this can be done in 5 minutes.

Now we are ready to measure the filter and check the bandwidth of both low and high channels on the scope. We will use a 1 KhZ .650 Volt sinus to start with.

But first we'll have to replace the old capacitors on the filter-board. Rogers used Tantalum capacitors that are awful for audio-applications but I guess they were fashionable in the 7-ties %-)

I'll take my bike and go to the electronics-shop. See you later.

In the meantime you should also take a look @ http://www.mhennessy1.f9.co.uk/rogers/ls58.htm

Revision Step 2: Checking the Filters

Revising and upgrading the 405-modules was straightforward, as I did about 100 of them before. Both amps (you have one per Rogers 5/8 speaker of course) have been made with +/- 4 years difference (1979 and 1982).

I think this is normal, the BBC ordered several hunderds of them from Quad/Rogers for on-the-road usage and stored them in the same warehouse. We can not expect similar pairs to stay together.

We are looking @ 2 405's with totally different Rogers-filters inside. Since 1982 Rogers removed the "Bass boost" option and simplified the schematic. The newer filter shows a strong distortion due to the input-transformer, which was to transform the symmetrical input to a non-symmetrical signal for the filter.

The Rogers-filters were built with transistors and very old components and there are no schematics to be found...

Revision Step 3: giving up the project...

There were many reasons why Jean-Marie and myself were very unhappy with the Rogers crossover filters.

Rogers used a small transformer to change the balanced input-signal to unbalanced in the input-line. Although this transformer does the job it causes a lot of distortion and phase-shifting.

The filter is made with transistors and some very old components, some of which were broke. The schematics can't be found.

So we decided to remove the filters, upgrade the Quad's and use a Behringer active filter instead.

Jean-Marie bought one for only 100 Euro. New, with a 2 year warranty. We'll connect it next week and we'll keep you posted...

What Peter Walker said about active crossovers (In an interview with Adio Amateur, 1978):

"AA: Some people like splitting up the signal with electronic crossovers, Do you see problems in that?

PW: It would cost more money! Provided you're not overloading, there are no advantages. If you've got three amplifiers covering three ranges, then you can run into overload with less trouble, because the bass can overload and it won't inter modulate the high. Therefore we can get away with more distortion audibly successfully. If you're saying that an amplifier should never run into clipping, then using three amplifiers is no better than one amplifier. One amplifier with three times the power would be rather better, because at least the power can come in any part of the frequency range. So a hundred Watt amplifier covering the lot would be better than three 30 Watt amplifiers, middle, bass, and treble. Because if all the signal comes in the middle, which it can do, then you're only limited to 30 Watts - the other amplifiers are sitting idly by and you'd like them to come along to the middle one's aid, and they can't do that."

You can read the full article on The Audio Circuit (http://www.audiocircuit.com).


Monday, January 30, 2006

Quad 405 Replacement Transistors

Manu wrote:

Peux-tu me donner les équivalences actuelles concernant les composanst suivants : - Transistor ZTX504 - Transistor ZTX304 - Transistor BC214C - Transistor 40872 ( sur ton site MJE15031 mais j'ai vu aussi BD244C ? ) - Diode IS902 - diode LR120C

Can you give me actual equivalents for (...) those transistors/diodes?
  • For the BDY77 power-transistor in the original schematic (TR9 / TR10), often replaced by the Toshiba 2SD424 in later models, I use the 2N3773. (http://www.onsemi.com/pub/Collateral/2N3773-D.PDF). This is a very"fast" transistor with excellent and overrated specs.
  • For the drivers 40872 (TR7 / TR8) I use - excellent - B536 from NEC. The 2SB546A should be an alternative but I did not test it.
  • ZTX304 and ZTX504 (several TR's) can be replaced by BC639 and BC640 resp. if necessary. Attention, the connections are different: B/E changed places. There is no reason to replace those transistors if they are not broke. (http://www.fairchildsemi.com/ds/BC/BC640.pdf).
  • Diode LR120C is a Zener that stabilises the Opamp power supply. If you replace the Opamp by a Burr-Brown or other modern chip, replace it for a 15 Volt zener. If you keep the old Opamp use a 12 Volt zener.
  • For the other Diodes any standard diode like the 1N4002 will do the job.
All Quad transistors & other components are available in the Dada Electronics Webshop.


Monday, January 16, 2006

405-revision Live ! Whodunnit?

Jan brought me his 405-2 to have it revised & upgraded. He and his father are classical-music fanatics.

Unfortunately, the right channel was broke lately. On top there is a note saying: "Right channel broke. 2 transistors broke. Short-circuit on the circuit-board near C5."

So I took my screwdriver and removed the top-cover. It is one of the latest 405-2's made (1982) - this is marked on top of the transformer.

All electrolytic capacitors clearly have to be replaced. 2006 - 1982 = 24 years. The best electrolyts have a life-time of 15 years.

We'll also have to replace the output-connectors with more modern plugs and we'll have to add RCA-inputs, leaving the DIN-connector in place, so it will be possible to use the amp with vintage Quad's as well as "modern" pre-amps.

But what killed the transistors?

Inspector Stefaan starts his interrogation today and will keep you posted.

Police report 1 - The pathologist's report

The Printed Circuit-boards were removed from the belly of the 405-2. The victim's-circuit board is on the left of the picture.

In the middle of the circuit-board we see some very old capacitors - that should have been replaced a long time ago.

The input-opamp is the TL710CP, already "vintage" in 1982 and soldered without plug-in connector.

For the input-circuit and the DC-feedback cheap carbon 5% resistors have been used.

In the middle of the board we can see surface-mounted electronics which are not on the schematics.

We have our first suspect: Quad from Huntington. Motive? Money, as in most crimes.

Police report 2 - The crime-site

On the back of the victim's circuit-board, a burn-spot beside C5 can be seen, indeed.

C5 itself, however seems in good shape.

What can have caused this damage on the backside of the circuit-board?

This leads us to suspect 2: an amateur who tried to repair it?


The problem solved!

We'll never know why, but the Printed-circuit lead was broke around C5, indeed. Maybe a production-error when the PC-board was made?

It burned after 20 years.

This lead connected the mass for the Opamp-zeners. The result is that when it broke the opamp had no mass-reference anymore.

In a 405 the output-DC voltage is kept @ 0 Volt by a DC-feedback line to the (-) input. As the Opamp had no mass-reference anymore voltage in the output-stage made strange jumps, killing TR7 and TR10.

Reconnecting the faulty lead with some solder as well as replacing TR 7, TR10 and the fuses solved the problem.

After replacing the Opamp with a Burr Brown OPA604 and all capacitors (the yellow one is a 47 mF bipolar for C2), Jan's 405-2 is up to 15 more years of excellent performance;-)


Sunday, January 08, 2006

Quad Revision FAQ's: Part II: The Quad 33

The 33 has a very beautiful design (to be discussed ;-) but also electronics from the 6-ties. Can it be upgraded to modern high-end standards?

Personally I love the 6-ties for the design in furniture, fashion, wall-paper and for the music. The fact I was 2 years old in 1960 & 11 years old in '69 must have something to do with it ;-).

6-ties design has come back in today's fashion as an icon of the best design of the previous century, beside Art Deco.

The Quad 33 was one of the best examples of 6-ties design. It was the first hifi-design to show that the highest quality does not mean the most buttons, a large size and impressive meters, lamps & switches on the front-panel.

But can the 33 be adapted to modern high-end standards?

Sure, it can.

The 33 is completely transistor-based. Later pre-amps used Opamps. Quad used them from the 34 on.

There is nothing wrong with Opamps. Only, the first opamps made in the 7-ties (like the LM301) did not match the specs of a well-designed transistor pre-amp. In the 34 the sound passes throught (at least) 4 Opamps, each containing (at least) 24 transistors. Later it got worse...

Less is more is the design rule for the best pre-amps designed today.The 33 respects this principle in a great way.

What can be done to upgrade my 33 to high-end standards?

Ruud Janssen's website is the best advice on the 33 (and the 303) - see the links on the left. Unfortunately it is only in Dutch.

What has to be done in any way is:
  • Replace all capacitors > 1 mF with new ones. They have dried out after more than 20 years.
  • Increase the powersupply-line from 12 Volt to 16 Volt (the reason it was 12 Volt is because modern capacitors would have been too big to fit in).

These are the basic things.

How about the Volume-attunator potentiometer of 50KOhm with a switch on it?

The potentiometer is 50K Ohm Log with a switch. This one is no longer available. A company in the UK sent me an offer on my request, but I had to order a minimum of 1.000 of them to start a production-line "for a small quantity".

The best alternative is a DACT-attenuator (a switch with 24 positions, stereo, logaritmic) that is applied in most modern high-end.

Anyway, count >100 Euro for this one (see http://www.dact.com/html/attenuators.html) and you will have to add an external switch. That's what I did.


To be continued...

Quad 33 Power-supply Voltage

In a previous comment today Hatif said:

"I'm interested in Quad 33-303 but I would need its mains supply changed to 100-120V in my country. Do I have to do some soldering or is there any selector on the unit? I see one on 303 poweramp but am not sure for 33."

On most Quads there is a voltage-selector switch on the back, but not on the 33.

For changing the mains voltage on a 33 one has to resolder the red wire on the left of the transformer to the middle connection (see the picture - or click it for a larger image).


PS. The 33 on the picture is not the "standard" 33. I only used the picture to show the transformer-connections.

In the 33 on the picture the volume-potentiometer is a DACT-attenuator, the output-circuits have been rebuilt and the power-supply changed to 16 Volt.

Monday, January 02, 2006

Quad 34 input sensitivity

Thomas had a (very usual) problem with the input-sensitivity of his 34.

He wrote:
"Als hij op CD-speler staat, moet ik hem half open zetten om voldoende volume te hebben. Op radio krijg ik hem dan weer niet stil genoeg (dus stand 1 is dan al te luid). De CD-speler is een quad 66, tuner voorlopig een ouwe marantz maar dat moet op termijn een FM4 worden."

"When it is set to CD, I have to put the volume half a way to get a reasonable volume. Set to Radio, I can't get it silent enough (position 1 is already too loud). The CD-player is a Quad 66, the tuner - for the time being -an old Marantz, but in time that has to become an FM4."

This is a typical problem with Quad-equipment, indeed. Quad defines "line-voltage" as 500 mV while most other manufacturers apply +/- 1,2 V today (and even at the time Marantz defined it this way...).

The standard input-sensitivity for the 34 is 300 mV (CD) and 100 mV (!) for the Radio (Tuner) input.

However, the CD-input can be easily adapted by changing R135/R136 to the correct value. The original Quad "flags" (see the picture on top) are not needed, good quality resistors (metal-film) will do. The Tape in- & outputs are adaptable the same way.

Unfortunately for adapting the Radio input some soldering is necessary as no variable sensitivity is foreseen by Quad.

The X1 / X2 "flags" are interconnections between the tape-output and the CD-connector (in order to use this one as a second Tape-connector).

The best solution for Thomas is to connect the old Marantz to the Tape-input I guess, adapting the sensitivity of it, until he has his FM4.

BTW. On the picture we also see the antique TL072 Opamps that should be exchanged for Burr Browns if we want the same dynamics "modern" endstage amplifiers like a revised 405 or 606 can provide.