Another Discrete Amp - GainBloak

[bcarso]

syn, if you have the tools to draft them you might consider showing a before and after schematic---it would be instructive for people learning troubleshooting. Just a thought.

 

[syn]

Sure, will be glad if this would be of any use to anyone... Graphics are of inferior quality due to conversion from pdf, sorry about that...

Some time ago i ran into this thread, and decided to give Bloaks a try. After assembling Big Bolak on the proto-board i was astonished how good it sounded. At that time I made no measurements, just compard it to some other home made DOAs, and have to say, bloak left very good (the best) impression on my ears. In the mean time i had a major computer crash, and the files that I've sent to the local PCB making shop were lost. I had no component layout, and for everything to be even worse, by mistake i handed them mirrored layouts so i got mirrored PCBs. I was not aware of that fact up to day or two ago. Add to that my level of knowledge, which is low of course, and you get the picture. MESS. The Big Bloak output stage was biased like on the picture below...

The sound was not as transparent, punchy AND fat as i remembered it. Everything was working,sound passed, no smoke, no heat, so at that time i was not suspecting the layout at all, thought I'm doing something wrong with DOA surrounding circuit...
Took some RMAA and wow, i had distortion figures as if i was running Bloak clipped to the maximum and then through the fuzz box. Then i decided to ask for help on the forum...

Tamas asked me if I'm using 4 diodes to bias the output, and a quick look at the Bloak reveled that they are biasing in the wrong direction. That was it I thought, replaced diodes with new ones and this time in the right direction, took some RMAA again and...well i got about 10 times better distortion figures but still 10 times worse then i hoped for...I was at this point at the time:

Samuel asked about quiescent current, googled and found out how to measure it. It was way belove desired. Quick look at the Bloak's traces
and i discovered another mess, BC550's collector was connected to the
MJE800base and BC560collector to the MJE 700 base. Cut the traces,
placed jumpers RMAA...yes this time Bloak gave figures as it was a
piece of wire. My audiophile 2496 is giving' almost same results when connected output to input.

Hope this helps to someone, and by all means DO make those Bloaks,
they ROCK...

 

[jeffrey_burr]

- Uses easy to obtain BJTs

Does this design criterium apply to the FETbloak also? I'm having a not-easy time finding 2SK170 of any flavor. Mouser and Digikey are my usual vendors. Is this part still in production? Perhaps there is an acceptable easy-to-source alternative?

 

[tk@halmi]

[quote author="jeffrey_burr"]

- Uses easy to obtain BJTs

Does this design criterium apply to the FETbloak also? I'm having a not-easy time finding 2SK170 of any flavor. Mouser and Digikey are my usual vendors. Is this part still in production? Perhaps there is an acceptable easy-to-source alternative?[/quote]

Those Toshiba parts are in still production. Linear Systems even makes a substitute called LSK170. In a pinch use a J113 or a PN4393. The pins are NOT assigned the same as the 2SK170!!!
Otherwise just stick the next N channel JFET in there you put your paws on while watching the pin assignment. It might suck. Then try something else.

 

[skipwave]

Maybe ask forum member guavatone where he bought from earlier this year. I got some from his batch.

http://groupdiy.com/index.php?topic=21547

 

[jeffrey_burr]

Gentlemen, thank you. LS is pretty close to where I live, maybe I can find a couple pieces that "fell off the truck"...

I did post a query in this same thread, a while back, and forgot to post again to say "thanks for being helpful". It was a stupid question anyway - all the more reason to be grateful I suppose.

I plan to follow the advice given way back then, and build a very simple transformer-coupled mic amp. Unless someone could present a good reason not to, I'll probably etch the whole circuit incorporating the opamp on one board. I doubt I'll regret not having a separate opamp module, and the pins look like a hassle. I found some diodes with 1.3V drop, in a similar package as the output darlingtons - I'd use two instead of the four regular 0.65V deals and mount all to a single heatsink. If this is foolishness, please do let me know.

And again thanks so much for sharing. For me this is all just to make me self-sufficient in the recording of my music, and I really appreciate all the helpfulness and generosity of the folks here!

 

[JohnRoberts]

[quote author="syn"]Sure, will be glad if this would be of any use to anyone... Graphics are of inferior quality due to conversion from pdf, sorry about that...

Hope this helps to someone, and by all means DO make those Bloaks,
they ROCK...[/quote]

Sorry I haven't followed this entire 14 page thread but from inspection this schematic looks like the output stage is better connected with the top NPN base to the top of the diode string, and the bottom PNP base to the bottom of the diode string. If this is addressed in the text somewhere else my apologies.

JR

 

[syn]

That schemo is wrong of course. It is just an example of what I’ve done wrong.

This is the correct one:

Apologies for any confusion this might have caused...

 

[skipwave]

[quote author="jeffrey_burr"]I'll probably etch the whole circuit incorporating the opamp on one board.[/quote]

If you like to etch, go for it. If you would consider a ready made solution, it would be Bloke PCBs from Tamas (pins included) and JLM's BabyAnimal PCB (with sockets).

That's what I did, and it made for a very simple build. Next up for me is a BabyAnimal Dual with a pair of FETBlokes on it.

 

[jeffrey_burr]

thanks for the suggestion, skipwave. I will take a look and seriously consider it. I'm not yet sure if I like to etch, but I am going to try it, with a very simple circuit.

I just got the transformers today on the ebay. a popular Jensen model. Can't stop now. I'll be happy if it's better than the ssm2019 in my Echo audiofire8.

 

[skipwave]

I forgot about ampslab:

http://www.ampslab.com/trans_2sk170bl.htm

 

[syn]

Can Bloaks be used unbalanced or "impedance balanced" at all? Whatever i do i have ~ 0.05-0.50Vdc after the output cap unless i connect transformer to it.Then there is no DC on the primary. Output cap 2200uF/50V (the same with 4700uF/50V ).
Thanks

EDIT:http://www.groupdiy.com/index.php?topic=26115
will probably give the answer to this Q.. I'll post my findings there.

 

[jaygunn]

I am ready to order parts for a couple of FET (maybe BIG) bloaks. They'll be used in API-like mic pre with Cinemag 75101APC input transformers (150:15K) and I'm planning to use 2SK170BL FETs. Output load will be 600:600 output transformer, OR Pico compressor-to-THAT1646-to-output transformer, depending on the setting of a bypass switch/relay (all in same box with very short leads). In either case on the other side of the output transformer will be the Line In of my Digi002R at 10K input impedance. Oh and I'll run the opamp at +-22-24VDC.

So I am not ever going to show these op amps a low impedance load. My impression from reading the 15 pages of this thread is that I don't need to run the bloaks especially high-current, since there's nothing challenging like a Pultec downstream from them.

Q1: Is there an adjustment you'd suggest for the current-setting resistor/diode (D7) or diodes D1, D2, D5, D6 to lower idle current down to 20-25mA to keep the opamp cooler?

Q2: Is there a benefit to NOT using the darlington transistors (also based on my not needing maximum output current)? Not too late for me to go either way.

final Q3: There was a lot of discussion about thermal connections between darlingtons (insulated bond to each other) and/or diode-transistor coupling. Is there a consensus about whether and what to couple if anything?

Thanks in advance...

 

[Samuel Groner]

I guess we're referring to the schematic linked in www.groupdiy.com/index.php?topic=27102?

Is there an adjustment you'd suggest for the current-setting resistor/diode (D7) or diodes D1, D2, D5, D6 to lower idle current down to 20-25 mA to keep the opamp cooler?

D7 or the according resistor should not be changed as it would change the collector current of Q2 as well which might have other implications. The other diodes don't offer any suitable change either (besides selecting them for low diode drop, but the 1N914B are pretty consistent so little to be gained from here). You could increase the emitter resistors R7/R8 a bit--increase the value in small steps (say 12, 15 and 18 Ohm) and observe the quiescent current.

Is there a benefit to NOT using the darlington transistors (also based on my not needing maximum output current)?

IIRC the version without darlingtons showed better stability.

There was a lot of discussion about thermal connections between darlingtons (insulated bond to each other) and/or diode-transistor coupling. Is there a consensus about whether and what to couple if anything?

Thermally coupling the diodes D1, D2, D5 and D6 pairwise to an output transistor will lower the quiescent current so this might be beneficial here.

Samuel

 

[skipwave]

[quote author="tk@halmi"]The latest one uses the MJE800/700 darlingtons. This reduces distortion by 75% through improved open loop gain, and drives impedances down to 75 ohms. It retais the 100V/uS+ slew rate. This one really consists of eight transistors, but it uses the same PCB as all the others.[/quote]

Apparently the darlingtons also yield lower distortion. I've built a handful of these things with the BD139/140 output devices. I'd be curious to compare and see if the difference netted by the darlington upgrade is audible.

Brian

 

[jaygunn]

The problem with trying out a project whose initial excitement has died down somewhat--the schematic and other links are often no longer functioning. This is from a previous page of this topic:

Here is a link to the latest documentation and the evolution of the Bloaks:
{Link that's no longer alive}
It includes drawings for:
- GainBloak
- FetBloak (2SK170BL input)
- Improved FetBloak (more stable VAS)
- The BigBloak (Darlington output transistors)
The latest one uses the MJE800/700 darlingtons. This reduces distortion by 75% through improved open loop gain, and drives impedances down to 75 ohms... This one really consists of eight transistors, but it uses the same PCB as all the others.

Does anyone have this documentation they could attach to an email for me? I'd be happy to host the docs on my own site for other Bloak builders.

I am planning to make two FETbloaks with MJE800/700 and two FETbloaks with BD139/140. For the Darlington version I have no idea where the "8 transistors" go; maybe that's from a superceded schematic? In the schematic on page 14 the Darlington version only has 5...

 

[syn]

Darlington version:

each darlington=2transistors=4 transistors
d7=1.1mA CS (fet+resistor)=1 transistor
input+vas+vas cs=3 transistor
----------------------------------------------
8 transistors

Vbias= 4x914b

Watch out, they get hot (darlingtons), use heatsink....

schematic: page 14, this thread, bottom of the page...

good luck

 

[skipwave]

I posted this in another thread for someone looking for these schematics.
[quote author="skipwave"]Yah..

Gain Bloke

Big FET Bloke

Email Tamas to buy some nice little PCBs.[/quote]

 

[Svart]

I am ready to order parts for a couple of FET (maybe BIG) bloaks. They'll be used in API-like mic pre

I'll just openly remind everyone that these are CURRENT feedback devices and that you cannot use the API circuit directly, you HAVE to remove the feedback capacitor and adjust the feedback resistors accordingly.

 

[TomWaterman]

IIRC, to add to Svart's comments.

TK recommended nothing more than about 15pF of feedback compensation as opposed to the 120pF in the API circuit.

Also I think Rf was 15k.

Somewhere he recommends a large lytic off Rg to ground to keep them stable and happy and DC offset low. I think it was 470uF? It was quite a bit larger than the API312 stock values anyway.

-T