TGM5 - all-BJT Simple Symmetric Amplifier

[Bigun]

I have settled on the use of multilayer ceramic caps (John "MJL211..."would be proud)

On the use of ceramic for Cdom:
http://www.diyaudio.com/forums/solid-state/169833-polypropylene-vs-silver-mica-cdom.html#post2236331

[…] i found also tha ceramic multileyer type sounded nice […]
regards sakis

C0G can be terrific for audio.

NPO is tolerable for audio[…]

[…] All in all C0G had the best value for money and size, ranked amongst best in my not serious subjective test, and yes, that makes them ''terrific for audio'' to me too, where their small values and voltages apply.

Maybe not the best you can get, but good enough, even for sub ppm distortion amps. edit: the higher the voltage rating, the less distortion etc.

From the datasheet for AVX C0G (NP0) ceramic caps:

"C0G (NP0) is the most popular formulation of the “temperature-compensating,” EIA Class I ceramic materials. Modern C0G (NP0) formulations contain neodymium, samarium and other rare earth oxides.
C0G (NP0) ceramics offer one of the most stable capacitor dielectrics available. Capacitance change with temperature is 0 ±30ppm/°C which is less than ±0.3% ��C from -55°C to +125°C. Capacitance drift or hysteresis for C0G (NP0) ceramics is negligible at less than ±0.05% versus up to ±2% for films. Typical capacitance change with life is less than ±0.1% for C0G (NP0), one-fifth that shown by most other dielectrics. C0G (NP0) formulations show no aging characteristics."

 

[Bigun]

A possible reason why LazyCat doesn't see an unstable dc-offset, whereas my simulations do show a touchy dc offset is the input impedance. The original SSA design has a 10k resistor at the input (to ground). I like to have a more common 47k resistor for higher input impedance. And this makes all the difference because if I drop my input resistor to 10k in the simulations the dc offset is much less temperature sensitive. There appears to be a happy 'middle ground', but at the extremes you either employ a dc-servo with high input impedance, or you employ a pre-amp, with low input impedance.

 

[Esperado]

A possible reason why LazyCat doesn't see an unstable dc-offset, whereas my simulations do show a touchy dc offset is the input impedance. The original SSA design has a 10k resistor at the input (to ground).

10K seems a normal input impedance for a power amp. Did your preamp is not able to drive 600 ohms of charge? Less is the lines's impedance, better it is too, for hum, noise, parasitic RF, and transients losses as you knows.
If your preamp is directly coupled, the amp will see the very low DC impedance source ( plus the one of any added serial resistance in the input), so i don't understand your fear with 10 K ? Unless your amp is cap isolated in input and you worry about cap value to avoid electrolytics ?

 

[Bigun]

Hi,

I'm afraid the day job and other items has been restricting my activities again to occassional visits to the forum. But progress is now made with ordering of parts....


p.s. I will likely build a pre-amp for this amp into the same box to buffer the volume control into the lowish input impedance of the TGM5.

 

[Bigun]

The parts have arrived, plus I have a final pcb layout and I hope to have time next week to have a go at etching the first one.

 

[SOUND_Rays]

SMD

The parts have arrived, plus I have a final pcb layout.

Bigun, You made the smallest board for this Amp. Beautifully looking neat small PCB
Can't wait to order the parts and try it also, can You make a list of the parts You order and the suppliers You chose?
It's really neat size, ... will nicely fit inside my actives as an TW and MID amp and with 2 OPs pairs
maybe also as Woof amp.
Waiting Yours impressions . . ..

 

[Bigun]

Hi,

I'm going to be a bit stuck for a couple of weeks with a lot going on at work but I will share my impressions when the first unit is running.

Final parts list may depend on tweaking etc. but I'll start by posting the final schematic used for the pcb layout.

I source all my parts from Digikey - keeps it simple.

 

[SOUND_Rays]

Hi,

I'm going to be a bit stuck for a couple of weeks with a lot going on at work ... but I'll start by posting the final schematic used for the pcb layout.

Excellent, can You post the SCH populated with values or TXT file with values chosen?
Thank You

 

[MiiB]

Bigun....

You seem to have double thermal control.. eg two time VBE.. why is that..??
or is the second string (T8-T9) error correction..??

 

[Pingrs]

Bigun....

You seem to have double thermal control.. eg two time VBE.. why is that..??
or is the second string (T8-T9) error correction..??

Isn't that a crude current limiter?

Brian.

 

[Lazy Cat]

Overcurrent protection, current limiter, you name it, it is a wise thing to have it when there is only one pair of outputs.

 

[Nico Ras]

Only if you plan wild drunken parties.

 

[Bigun]

Some wild parties would be nice !

Yes, I included an optional current limiter. It's not intended to protect the output devices from naughty speaker loads. It's there to give some chance of survival in case of an accidental short whilst I muck about with it. It's designed to stay out of the way - it's formed with an RC filter so that it offers protection at subsonic frequencies, i.e. it's NOT a VI limiter.

SOUND-Rays, parts list is something that may change - after all I haven't shown that this thing works yet.

Resistors - 1% 1/2W Stackpole Industries
Electrolytics - Polymer low ESR

C1 1n NP0
C2, C3 1000u 16V
C4 100u - eliminate for increased 2nd harmonic
C5, C6 5p6 NP0 (Cdom - may not be needed, value determined by 'ear' and CRO)
C7, C9, C14, C16 220u 63V
C8 33u
C10, C13 47p NP0
C11, C12 33u
C15, C17 100n Film
D1, D2, D3, D4, D5 1N4148
D6, D7 1A 1000V ('catch' diodes)
F1, F2 4A Fast Blo
R1 1k
R2 22k
R3, R4 220
R5, R6 10
R7, R10 220R
R8, R9 10R
R11 1k8
R12 1k8
R13 2k2
R14 2k2
R15, R16 4k7
R17, R18, R21 47R
R19 1k
R20 330R
R22, R23 100
R24, R25 47R
R26, R27 280R
R28 100R 1W 5% zobel
R29, R32, R35 10
R30, R31 tbd
R33, R34 0R33 1% Dale 2W
R36, R37 (not shown on schematic) 100R 1W in parallel with rail fuses
T1,2 MMDT5451 (single package, two transistors - tiny)
T3 MMBT5551
T4 MMBT5401
T5, T12 PBSS9110Z
T6, T13 PBSS8110Z
T7, T8, T10 MMBT5551
T9, T11 MMBT5401
T14 TO-264_NPN (pick your favourite)
T15 TO-264_PNP (pick your favourite)
TRIM1 50k (dc offset)
TRIM2 500R (idle current for output devices)
Z1, Z2 BZT52C1213 (12V)


Input capacitor not included.

 

[SOUND_Rays]

Some wild parties would be nice !

Thank You, now plenty of time for searching the parts, may I suggest You for sonically the best sound available they'd be the Exicon ECX10N20 & ECX10P20.

 

[MiiB]

hmmm... still up with the feds...ehh fets, some wild party in an all BJT..thread..

 

[SOUND_Rays]

Dear MiiB,

It's only intend as a suggestion, nothing harmful at all, and
the sound this way could be pleasant too ..., I'll try both BJT
and MOS version for sure, because each have its specific
qualities, and then only ears will judge ...

 

[Elvee]

Final parts list may depend on tweaking etc. but I'll start by posting the final schematic used for the pcb layout.

Before committing to the PCB, you could streamline the schematic: some of the components are essentially useless, and could be eliminated, or reused to improve your input stability problem.
The cascoding brings no benefit here, which means T1,2 can be dispensed with, by entering directly on the bases of T3 and T4.
Eliminating those superfluous components should slightly improves things, or at the very least be neutral.
And if you want to keep/reuse T1, 2, you can use them as a diamond input buffer: they will raise the input impedance and thermally compensate T3 and T4, which can only be beneficial.

 

[Ian Finch]

The answer to your request Sound Rays, is in the thread title ......All-BJT ..... It would be crazy to include what has just been excluded by the title.

 

[SOUND_Rays]

The answer to your request

Ian,

It was only a suggestion to try, not a request at all . .

 

[Lazy Cat]

Before committing to the PCB, you could streamline the schematic: some of the components are essentially useless, and could be eliminated, or reused to improve your input stability problem.
The cascoding brings no benefit here, which means T1,2 can be dispensed with, by entering directly on the bases of T3 and T4.
Eliminating those superfluous components should slightly improves things, or at the very least be neutral.
And if you want to keep/reuse T1, 2, you can use them as a diamond input buffer: they will raise the input impedance and thermally compensate T3 and T4, which can only be beneficial.

What a joker you are ...