modify my old 80s amp

[lineup]

Re: Post 13

Hi.
What is the function of the R7//C4.
Thank.
Geirin.

R7 is quite often seen in good amplifiers.
But likewise often not in many amateur amps.
Usually a quite low value resistor.
What I have understood, it gives better control of the output stage Bias Current.

U4, BD139 senses the heat.
And as transistors get Volt Base-Emitter ~2 mV lower / 1 degree C increased temperature,
BD139 will reduce the voltage between Output Base-Base,
when heatsink get's hotter.

If heatsink get's 40 C hotter, the Voltage b-e of U4 will be ~ 80 mV less.
This difference is multiplied by (R8+R13+R18)/R18.
Which is like x4.
Result is:
Temp +40 C = - 4x 80 mV = -320 mV,
between Output Base-Base.

Also those both Output big transistors will have less Vb-e.
And if this is the same value as -320mV,
then the average voltage across R9+R15 will stay almost the same.
We have an ideal temp compensation, to avoid thermal runaway
and get the amplifier overheated.
------------------------


For some reason this Vbe multiplier temperature compensation is not 100%.
Not without such little resistor.

About C4 which is across the R7, 47 Ohm.
I have not noticed this cap in amplifiers before, but ..... as I can understand:
As the temp compensation is DC Level Thing,
the C4 makes R7 'more invisible' for AC signals,
so that R7 does not effect the higher frequency Audio signals, hardly anything.

 

[destroyer X]

I wquld remove those caps and listen to the amplifier...

only... and i think it will sound sligthly better...well... it is already a good sound amplifier.

regards,

Carlos

 

[GEirin]

Hi Lineup.

Thank you very much for your detailed explanations.

GEirin

 

[lineup]

Hi Lineup.
Thank you very much for your detailed explanations.
GEirin

why this little simple saying makes me so happy: tank u

... really do not know .. or maybe i do
anyway.
have never costed me one € Euro or one Swedish crown SKr
to express my gratitude
...and give away my Thank U Veru Muchos Gratias

GEirin 🙂 my friend
Send my respect & honor to your parents.
If they are alive.
They sure did a good job.
With you, their young child.

------------------------------------------------


There are some things, among we people,
that are far far more important, than post some diy audio ideas & knowledge.
😎 This is to spread the respect and goodwill to wards other humans 😎
... instead of contribute into this world
your war,
troubles,
your suspect suspicions,
your misunderstandings & intolerances and your deaths & sorrow

Lineup - sweden june 2008

 

[ECBLN]

Hey guys !

sorry i´m late! cause , i need much time for my diploma thesis ...

- but the measurements on the circuit will come soon as possible...

by the way, the amp is working already with updated parts, and it sound´s very nice !!!

bye & best regards

ECBLN

 

[henrique]

Hi everybody!

I happen to also have one old Grundig SV2000 and I'm trying to restore it.
Only today I was able to test it for the first time but the right speaker doesn't work. I think the problem is not the speakers, I've switched them and and the problem seems to be the amplifier.

Does anyone have the full schematics?
Also does anyone have a clue of where should I start looking for the problem? (my first guess is that one of the amplifier circuits is not working)

Best regards

 

[wahab]

The Darlington output is already changed to BDV64/65 type.
(they are similar to the special GP-type´s from Grundig)

Although working well generally , BDV64/65 pair
is out of date performance wise for audio purposes.

Sanken s MP1620/MN2488 would be far better.
2SB1647/2SD2560 , 2SB1670/2SD2401 and
2SB1648/2SD2561 fit the job as well...