diyAB Amp - The "Honey Badger"

[Variac]

I have asked member ostripper to lay out a Class AB amp officially called the "diyAB amp" ( nicknamed "the Honey Badger" ) using many of the best characteristics of amps of this type that have appeared on our forums, so that we can feature it in the diyAudio Store. He has agreed to give it a go, is really interested in comments to help it be a paradigm of its type, and will be posting here very soon to describe where the design is at this point.

So pile on!

Mark
"Variac"

Build thread for diyAB Amp: http://www.diyaudio.com/forums/solid...ld-thread.html

[ostripper]

After corresponding with Variac on the present layout of the DIY case they will offer and the diyAudio heatsinks drilling pattern, I have ported the well known TMC LIN amp to a 75mm X 250mm PCB (below 1).

Some doubt the level of fidelity a circuit such as this can reproduce , but I aim to show them that simplicity can be good.

The design goals ( correct me if I'm wrong) are :

- 150W/8R - 250+W/4R , 3 pair easily obtained BJT output devices run in a Self EF2 configuration. (to-3p or to- 247)

- "All in one" construction - A pair of Apex Jr. 8200uf's , or just about any 10mm or 22mm 4 pin snap-in capacitor can be mounted on the main board. I looked up the panasonic line of 35mm caps and one could even grace this amp with 22,000uf X 2 @ 63V.

- To avoid some of the layout "issues" in common with some DIYA amps. Grounding will be fully starred with voltage stage/input stage "lifted" and brought to the star at center point. I could also implement separate or "boosted" supply points which could be used if desired. Output feedback take off point and speaker tap are also "dead center". All semi's will be thermally coupled for almost "military" reliability.

- Compensation and other aspects of the basic design will be user defined (miller - tmc - 2 pole) , I have tried all 3 with success. The basic design is quite documented already in both D. Self and B. Cordell's books. My oldest DIY amp is similar and has been used for 2 1/2 years.

-Cost , this would be up to DIYA - There are no "exotic parts" , all are readily sourced. Even as this is so , the amp generally will have -110db noise , 1-2mv offset , and near PPM distortions with 80V/us slew.


Any comments or suggestions , the base circuit is (below 2). Any valid idea's on either the circuit or layout welcome. I am at the voltage stage / input stage layout now - I think the output stage will be free from any errata - I've used this "style" before .
PS - the board will be double sided with the option of a single sided full DIY etch (no jumpers).
OS

[pinkmouse]

Hi OS, the only things I would suggest adding are onboard DC and overcurrent protection, make things nice and simple and bombproof for newbies. You could always allow the option of linking out if not required.

[Tekko]

Whats with the batteries in the schematic ?

[Bigun]

Fantastic idea !

Please make it easy to measure the bias voltage across the output device emitter resistors without blowing up the amp with 'slip' of the multimeter probes - you can guess why I want to see this feature !

[5th element]

I would have thought that over current protection would be mandatory. The only issue I've got is that the transistors in the schematic (mainly small signal) aren't readily available to those in the UK. I have used them all myself though, but that required an order to digikey/mouser. Although if you can show me where to buy 1845/992s in small quantities without mad high shipping costs, that would be great.

[ostripper]

Originally Posted by pinkmouse

Hi OS, the only things I would suggest adding are onboard DC and overcurrent protection, make things nice and simple and bombproof for newbies. You could always allow the option of linking out if not required.

That would be nice. I am constrained by the case/heatsink configuration. A protection circuit needs it's own supply / relay / driver. After reading about member Bonsai's experience , a standard relay driven by a standard ta7317 WILL NOT always work with a high current amp. PMA's HEXFET solid state solution will handle 100's of amperes and disconnect in a couple uS's. Perhaps we can design a "deluxe" unit . A breakaway board with softstart/timer and ta7317/SS relay. DIYA could "corner the market". I agree this is a shortfall in many DIY amps.. but some audiophiles don't like a relay at the output regardless (OPT-IN is always better). I could integrate but only at the expense of the huge supply caps.

By tekko - Whats with the batteries in the schematic ?

The batteries are to manually DC bias the input stage , LT's capacitor models "cloud" the FFT data with artifacts. Keentoken enlightened me to this condition.

By bigun -Please make it easy to measure the bias voltage across the output device emitter resistors without blowing up the amp with 'slip' of the multimeter probes - you can guess why I want to see this feature !

A very easy to add convenience feature .... those 22mm ceramic .22R resistors do measure easily , but a set of extra pads with standoff pin's (tp1 - tp2) would be nice.

By 5th element - I would have thought that over current protection would be mandatory. The only issue I've got is that the transistors in the schematic (mainly small signal) aren't readily available to those in the UK. I have used them all myself though, but that required an order to digikey/mouser. Although if you can show me where to buy 1845/992s in small quantities without mad high shipping costs, that would be great.

Take a look at the Krell 300i below (no protection). I have mixed feelings about this , some from actually doing the "deed" ! This amp (the lin) typically just opens the fuses when shorted at even high- moderate volumes , I have only taken out the output stage with a slipped screwdriver or probe ( me too, - Bigun).

To address the sourcing , I'm sure we could find a way to include the fairchild semi's. Worst case , A970/C2240's can be used for the input stage , any 200+Hfe / 120Vce/ BCE device would give similar performance. MJE340/350 can be used for the VAS , but with a slight performance degradation. ANY to-3P or to-247 can be used for the output stage ( 60-120 Hfe/ 200v+ Vce). 2sc4793/2sa1837 can be used for drivers , with dedicated 8R use.
You could literally fill the BOM from your junkbox , and still have a pretty tight spec'ed amp.

OS

[Bigun]

You may already be planning this - but if not, I'd suggest adding AKSA-style resistors in parallel with the rail fuses. They have proved helpful to me in the past.

On the schematic posted above you seem to have used the same ground symbol throughout (unless I've read it wrongly), I think you want to have the protective earth / ground lift go to the chasis instead - you probably intended this but I would normally have moved the ground symbol to signal input ground even in a simulation so thought I'd just mention it.

I defer to your better experience, but I usually plan to have the output choke off-board, wire wrapped resistor in line with speaker wiring - keeps the magnetic field lines away from sensitive parts of the circuit. I don't know if this is important or not, just something I do.

p.s. I like the long aspect ratio of this board.

[farjon]

Originally Posted by pinkmouse

Hi OS, the only things I would suggest adding are onboard DC and overcurrent protection, make things nice and simple and bombproof for newbies. You could always allow the option of linking out if not required.

Yes... about VI limiter? Just leave protection components out if desired.

What about fuses onboard with parallel resistors a la Aksa 55 and Baby-Aksa, to measure bias current?

[ostripper]

Originally Posted by Bigun

You may already be planning this - but if not, I'd suggest adding AKSA-style resistors in parallel with the rail fuses. They have proved helpful to me in the past.

On the schematic posted above you seem to have used the same ground symbol throughout (unless I've read it wrongly), I think you want to have the protective earth / ground lift go to the chasis instead - you probably intended this but I would normally have moved the ground symbol to signal input ground even in a simulation so thought I'd just mention it.

I defer to your better experience, but I usually plan to have the output choke off-board, wire wrapped resistor in line with speaker wiring - keeps the magnetic field lines away from sensitive parts of the circuit. I don't know if this is important or not, just something I do.

p.s. I like the long aspect ratio of this board.

OK - you are a bonanza of idea's, Bigun

If you look below , you will see the "TP1" and "TP2" , if you can't probe those - stay away from DIY

The fuse resistors are there , too. I was thinking , is it not the purpose of these resistors to keep the VAS/input stage "alive" in case of fuse failure ? If this is the case , would it not be better to have the "safety resistors" run directly from the unfused faston to the small signal section of the amp ?? On my modular creations , I use 22R fusible resistors on the boosted VAS/IPS board. These double as both the R/C filter and as a safety factor.


The grounds are denoted as "G1" and "G2". That is for my modular and "super" amps that use separate power supply PCB's. On this integrated supply board , G2 would be a separate trace that meets the center of the main star. From that trace , the 5-10R "lift" resistor + 2 diodes and the other input stage earth references are derived. I directly reference the DC cap and VAS ground to the earth BEFORE the lift resistor. If you reference them AFTER the resistor , you have modulation of your reference. Comments on this are welcome.

I really have thought about protection. On the typical OEM with just adequate capacitance and current limited trafo's , the classic 7317 IC/10A relay will protect (in most cases). Here we will have up to 20Kuf X 2 and most likely a hefty 400VA X 2 or 800VA+ toriod. Just ask Bonsai about his blown speakers and melted relay .

To monitor TP1 and 2 with a SOTA DC/current detection IC and output this to a MUCH faster solid state relay would be better. If TP1 or 2 reached a preset voltage for X milliseconds (Rod elliots PIC detector or similar)Project 111 - PIC Based Speaker Protection
, it could fire the relay within a couple uS.

On board VI limiters will always affect sonics , as well. MUCH more defined and scalable VI limiting can be done with an external circuit as I described.

Variac discussed 2 PCB's originally , The amp itself just needs one , PS and all. A comprehensive VI/DC/soft start would be a good solution to all the above factors. It could also be ported to any amp.

Sound buster , the safety resistors can be used to measure OP current + the
current demands of the input circuitry. To measure the output Re's directly is more accurate and you can see if any OP devices are failing to "share" (gross Hfe mismatches). Most ON semi output devices from the same order will typically read within a mV across one of the .22R resistors. For example , these amps are typically set at 10-12mV.

OS