Showing posts with label DIY. Show all posts
Showing posts with label DIY. Show all posts

Monday, 11 August 2014

TTSH (ARP 2600 clone) build notes - VCA (DIY)

The VCA build of the TTSH synthesizer. 

This is part 9 of my TTSH (Two thousand six hundred) build which is an ARP 2600 clone synth.
You can see my full build thread HERE

There are two awesome TTSH threads in Muffs.
2600 clone - Two Thousand Six Hundred (TTSH)
and
Two Thousand Six Hundred (TTSH) Project General Build Thread

The DSL-Man site has a very good TTSH thread

And of course the official build thread is here



The ARP 2600 used 4010 & 4019 sub-modules in the VCA section.
There were matched transistors (TZ-581 & TZ-81)  and a Op Amp (LM 301) at the output stage.

The TTSH uses the TZ-581 & TZ-81 modern equivalents (2N3904 & 2N3906)
The LM301 is unchanged. :-)
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 Front PCB (pre population)

Back face of PCB

 Resistors, Diodes then Caps.

Unmatched transistors.

Matched transistors & the LM301 op-amp

Thursday, 7 August 2014

Transistor Matching.

This page is part of my exploration re transistor matching.
Many synth circuits require matched pairs of transistors... often for differential amplifiers.
There are lots of transistor matching circuits and devices out there.
Many ways...... but the question is which to use. ???

Vbe (base-emitter voltage) matching is the most common type of matching done. It is what is used for matching transistors in an exponential converter for example. It can be done with a circuit like Ian Fritz's, or can be roughly done using the diode setting on a volt meter (Ian's circuit gives a more accurate result).
I like using a Atlas DCA Pro DCA75  




Why choose to measure Vbe ?
Taking a NPN transistor as an example, the main point is that the current passing through the transistor depends on the Vbe, the Ies & the temperature (Vt). This is according to the Ebers-Moll model:
 
Ie = Emitter current
Ies = the reverse saturation current of the base emitter diode.
Vbe = the Base-emitter voltage
Vt = thermal voltage

Ies depends on how the transistor is made and can be different from one transistor to the next.
It also changes with temperature.
The goal of transistor matching is to find pairs or transistors with the same Ies.

It's very difficult to measure Ies, but we can instead measure the Vbe if we pass a known current through the transistor. If we can keep the temperature constant, then matching Vbe is equal to matching Ies.

So the solution (if you are to build a circuit) is to have 3 things:
1. an accurate current source.
2. an accurate multimeter.
3. a constant temperature.

Whether you are using a circuit or the Atlas peak, remember that the temperature of the transistor will effect the measurements. I like to leave the transistors alone for about 5 minutes after handling them
and them carry out the measurements.
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The TTSH build required matched transistors in the filter section.
I matched them via hFE ... not by the usual the Vbe (base-emitter voltage).

I'm hoping I'll get away with it for that instance. However, the TTSH VCA section requires more matched trannies. So it's time I guess to bite the bullet.

Moog has a classic transistor checker schematic (from the mini-moog manual).
Here one current drives another current through the transistors and you need to measure the voltages between the base & emitter. It calls for a +/- 10V supply and needs a really accurate volt meter.

There is a variation of this circuit on DragonflyAlley.com
It uses a +/-15V supply, a 741 op-amp & 3 resistors.

 The Op-amp is a 741. (Pin 6 of the 741 connects to the base of the 3904)

 Here are some pics of the breadboarded circuit. (just for the NPN 3904 trannie).


It seems to work though there is lots of voltage fluctuation
Very temp sensitive.

Ian Fritz also has a transistor matching circuit.
You can download it here.

This is a breadboard of the Fritz transistor matcher.
The crossed resistors need to be matched as closely as possible. I guess a variable trim-pot could also be used to get a closer matching between the resistors.

I kinda like it more than the Moog circuit. It's definitely much simpler using just 3 resistors & a diode.

The voltage on the multimeter is the difference between the two transistors. In an ideal world of perfectly matched resistors & transistors the voltage would be zero.

This particular batch of 3906s seem to vary by 0.5 to 4 millivolts.

For completeness here are a few more links to blogs, websites which discuss transistor matching.
Hopefully you will find this useful.

Muffs has a cool thread for DIY Transistor matching if you finally wish to go down this path..
matching transistors - DIY
 
The famous minimoog tester of Dr. Robert Moog.

MFOS has a great page on how to build transistor matchers.
(MFOS Practical Transistor Matching)

I might try this MFOS matcher later if the earlier attempts aren't successful.

Links
+ Transistor - Basic info
+ JFETS - matching for synthesizers  
+ Muffs
+YATMA Yet Another Transistor Matcher

Friday, 1 August 2014

TTSH Synth - Two Thousand Six Hundred - 4020 ADSR (DIY)

The envelope generator build of the TTSH synthesizer. 
This is part 8 of my TTSH (Two thousand six hundred) build which is an ARP 2600 clone synth.
You can see my full build thread HERE

There are two awesome TTSH threads in Muffs.
2600 clone - Two Thousand Six Hundred (TTSH)
and
Two Thousand Six Hundred (TTSH) Project General Build Thread

And of course the official build thread is here
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The ARP 2600 has two envelope generators: an AR & ADSR.

 The EG section of the TTSH faceplate.

Vintage 2600s use a 4020 submodule in the ADSR section.
The TTSH synth has this built into the main PCB.

These are pics of a vintage 4020 submodule from an old 2600

The large silver circular component is a LM301

The TTSH uses the modern version.




Trying to match up the TTSH BOM with the old schematics of the 4020.... they are pretty close.
(I've read there were some (intentional?) errors in the 4020 schematic... might be just a rumor though).

The TTSH BOM of Integrated circuits & semiconductors is:
       1N4148 x 8 (CR1-CR8)
       2N3904 x 1 ???
       2N3906 x 3 (Q1,Q5)
       2N4392 x 1  (Q3)
       2N5172 x 3 (Q2,Q6,Q7,Q8)
       2N5460 x 1 (Q4)
       LM301 x 1 (A1)

 Resistors
  • 120 x 1 (R20)
  • 1k x 1 (R12)
  • 1k5 x 1 (R24)
  • 4k7 x 1 (R15)
  • 9k1 x 1  ??
  • 10k x 2 (R7 & ?)
  • 12k x 1 (R23)
  • 15k x 1 (R5)
  • 22k x 1 (R2)
  • 33k x 1 (R8)
  • 39k x 1
  • 40k2 x 1 (R11)
  • 68k x 2 (R17 - 19)
  • 68k1 x 1 (R13)
  • 100k x 2 (R14 & ?)
  • 120k x 1 (R10)
  • 180k x 2 (R4,R22)
  • 270k x 1 (R1)
  • 470k x 1 (R18)
  • 1M x 2 (R6,R9)
  • 1M2 x 1 (R3)
In the ADSR section, at the input of the LM301 (A1), the resistor going to GND [R7] is 10K in the original schemo. The TTSH uses a 33k instead.


A work of art.


Maybe the envelopes on the 2600 could be improved ???
Without modifications, their maximum Attack, Decay and Release times are short (esp the attack ---> 1.5 seconds on mine). The Decay & Release were about 10 secs. This could be a good place to start future modifications.

Phil Cirocco of Discrete Synthesizer offers EG upgrades to 2600s.
He describes the ADSR in a stock 2600 as "quick".
His upgrade looks cool:

"ADSR: The ADSR is quick in a stock 2600. The 3 position switch we install gives you a x1(factory speed), a x2 and a x3 speed, providing much longer times previously unavailable on a 2600.
AR: Conversely, the AR generator in a 2600 is quite sluggish in certain situations. This mod now includes an upgrade of the AR circuit. In addition to the normal X1 mode, the high quality 3 position switch provides a new high speed mode for sharper attacks, as well as a X2 mode for longer attack and release times."
Anyway, below are pics of the TTSH (envelope section) PCBs before population with most of the components:

 The rear PCB.

The front PCB.

For now, I'll build the EG section to TTSH specs.

Resistors & diodes first as usual.

Caps next. The TTSH schematic uses a 30pF cap here in The AR section.
However the BOM specifies the 33pF cap from Mouser.
Hopefully it won't make too much difference.

The  2N5460 is a P-channel JFET.designed primarily for low level audio and general purpose applications with high impedance signal sources. In the TTSH it's used in the AR envelope section. 
It was also used in the early ARP2500 1004 VCO module.

 AR section. Two 1M sliders & the SPST Off(On) Pushbutton Switch.

The 2N4392
N-Channel JFET used in the ADSR section.It's identical to the JFET used on my original 4020 module.




The Decay slider in the ADSR is 100K linear. The rest are 1M Audio.

All components are in. Ready to test.


Tuesday, 29 July 2014

TTSH Synthesizer - Two Thousand Six Hundred - 4012 Filter (DIY)

The filter build of the TTSH synthesizer. 
This is part 7 of my TTSH (Two thousand six hundred) build which is an ARP 2600 clone synth.
You can see my full build thread HERE



I understand this filter is based around the coveted Model 4012 filter, a four-pole low-pass VCF.
The 4012 filter used a design that closely resembled Moog's ladder-filter. This led to a legal dispute between Moog and ARP which forced ARP to design a new filter for their later models.
All the early ARP 2600 (pre 1977) used this filter. It's one of the reasons why the early Blue Marvins,  Grey Meanies, the (pre 1977) 2600s & 2601 v1.0 sounded so good.

The original design uses 8 pairs of matched and thermally coupled TZ-81 and TZ-581 transistor pairs.
There is a Dual FET AD3958, a LM301 Op Amp & temperature compensation via a 1K87 tempco resistor.

The TTSH combines the filter sub-module with the additional main board filter circuitry. 

Post 1976, ARP used the model 4072 filter (which was in turn replaced with the ARP 4075 4-pole 24dB low pass filter in the ARP Odyssey Mark 3, Omni and Quadra).

Some pics of the almost virgin PCB first:

 This is the back of the PCB

And the front section

Resistors & diodes first as usual.

These are Multilayer Ceramic Capacitors MLCC - Leaded 47pF 50V.
The BOM specified 50pF (as does the original 4012 schematic) but the Mouser order number (810-FK28C0G1H470J) shows them as 47pF.
I hope the slight discrepancy won't cause problems.

Caps & headers in.

The original 4012 filter used 8 pairs of matched and thermally coupled TZ-81 and TZ-581 transistor pairs.
The TZ81 was a NPN made by Sprague.The TZ-581 is a PNP trannie

The TZ-81 is now defunct. The TTSH uses the 2N3904. (its modern equivalent).
The TZ-581 is also defunct. The 2N3906 is a modern equivalent.


This is what the PCB looks like before the transistors:

Some of the trannies need to be matched.
First insert the unmatched transistors:

 
 This is a 2N395. It's a dual JFET


2N395 pinout

 Unmatched transistors.

There are lot's of ways to match transistors.
Vbe (base-emitter voltage) Vs  Hfe (current gain). ????

And the question of whether its really needed in this case is debatable.
Modern manufacturing tolerances are far tighter than back in the 70s.
I'm buying all my transistors in a single order so they should come from a single manufacturing batch. I guess that matching will only make a difference when your trannies come from several sources.

I'm testing each PCB as I go along & … I'll swap for matched pairs at a later date if I need to.
So far I've not run into any problems, but I found that thermal coupling (
thermal compound between the transistors) seems to help.

Anyway, Muffs has a cool thread for DIY Transistor matching if you finally wish to go down this path..
matching transistors - DIY
 
Vbe (base-emitter voltage) matching is the most common type of matching.
Eg: the famous tester of Dr. Robert Moog.
Moog assumed you have a +/-10V supply. He measured the base to emitter voltage.
You need a volt meter capable of reading to 0.001Volts DC.
(Sadly my VC97 meter only does 2 decimal points. ... time for a new one I think)

MFOS has a great page on how to build transistor matchers.
(MFOS Practical Transistor Matching)

At the moment I'm using a cheap digital multimeter. (Vichy VC97) with a a hFE mode.
(It's useless for Vbe matching).



Matching hFE is really easy to do. First define whether your trannie is NPN or PNP,
 then insert the emitter, base & collector into the appropriate hole.The DMM calculates the hFE for IB (In fractions of a mA I think). The value will be displayed on the LCD.

IB = base current
IC = collector current
hFE = DC current gain = IC / IB
The hFE varies with the current draw, the temperature & the applied voltage ... so I guess if we want to be really precise we should measure this all under the circumstances the transistor will see in its working environment.

These are my 3904s sorted in order of hFE

Same with the 2N3906s

Tempco 1k87 resistor.

The transistor ladder of hFE matched 2N3904s

Trimpots finally.