Transform Low to Hi impedance at audio frequencies

[Quarlo Klobrigney]

3035
https://nvhrbiblio.nl/schema/Grundig_3035WF.pdf


Either input makes it's way to the EABC80 - 6T8 grid.
I applied the signal to pin 8 and the volume is much louder, but of course, thin without the negative feedback and tone shaping circuits.

[Quarlo Klobrigney]

Simplified, it looks like this.
3 dots are an SPDT switch example 10.1 10.2 and 10.3, 11.1 11.2 11.3 with the .2 being the wiper. L is treble, B is bass.
C64 being the coupling/iso capacitor.
A switch, ACB passing on the signal through the Bass pot and C63 to the top of the Treble pot.

[Quarlo Klobrigney]

TA = Phono in, TB Tape in.
Both look like 1MΩ loads.

[bdunham7]

How does the radio sound?

[Quarlo Klobrigney]

Great like new after much rework.

No "let's try and see what it does" by plugging in first even with light bulbs or Variacs.
Straight on to visual inspection, change the ECH81 after 60 years the're all knackered, R&R of the selenium rectifier, re-capping completely, then resistor checking, alignment etc.


Never have I had one give me any trouble in the years I've owned the fleet.

[bdunham7]

OK then, can you measure the output of your BlueTooth module with a scope?

[Quarlo Klobrigney]

Can do, and the resistance of each channel.....Tomorrow as I get too tired too early now.

[David Hess]

A transconductance amplifier converts its input into a high impedance output.  A high impedance output is the same as a current source.  The circuit you probably want is a Howland current pump.

[Quarlo Klobrigney]

Next......

[Quarlo Klobrigney]

Thanks I will look into that amplifier as the BT output is low.
 
Feeding the BT output directly into either back input forces me to raise the volume control past the loudness tap thereby thinning out the fidelity.
The only thing that almost gives me an appropriate volume (my hearing is fine) is feeding the output into the top of the volume port.
But, there goes the switched input, the EQ and level.

============================================

Wait....I had no output, and I'm talking about audio output levels.

What, after some many hours....

Let's go to the original setup.
If you look at the BT module photo 1, I soldered a small piece of Teflon silver-plated 2 conductor shielded cable to the L-R output pins and the shield ground to the USB.
Then I mono out the radio end via 2 10Ω resistors so at least there is no phase cancelling.

Here's where everything went wrong.
After much tearing apart of that hookup, I found that there was a direct short of 10Ω to ground on the radio cable end.

I always break the problem in half........
It must be the cable, removing the resistors and unsoldering the cable from the BT output pins still left a short on only the left pin.
I had no over soldering to the ground plane.
 
I thought it must be the crappy 3.5 mm jack. It was removed with prejudice and some diagonal cutters.

I still had the short. I backtracked the L-R output pins and found the the audio jack interrupts the audio to the pins.

The short was in the board!

Scraping the board with a razor blade (the board is dead to me at this time) revealed that the ground-hold down pin on the 3.5 mm jack was shorted to the L output pin from the factory.
I removed that short.

The BT output comes directly from the IC via 2x 10µF 16V capacitors to the 3.5 mm jack. The 2 capacitors were outputting audio just fine.

After the butcher job I performed, I replaced the BT module with another, thus giving the levels as described in the 2nd paragraph.

I still need an amplifier, but not for the whispers or as drastic as a high gain low to high impedance transformation type.

Quote from: David Hess on Today at 04:17:24

A transconductance amplifier converts its input into a high impedance output.  A high impedance output is the same as a current source.  The circuit you probably want is a Howland current pump.

[TimFox]

A transconductance amplifier converts its input into a high impedance output.  A high impedance output is the same as a current source.  The circuit you probably want is a Howland current pump.

A cuirrent source (such as the Howland pump) has a high output impedance, but a transimpedance amplifier (op amp with feedback resistor) has a low output impedance.
I don't recommend either for his actual purpose.

[Quarlo Klobrigney]

I've been knocked out most of yesterday.
No Bluetooth to deal with but I did manage to determine if I inject the signal at any point in the audio chain other than through a switched input, the low impedance shunts the audio from the radio sources.


I've got to find a way through (back to this again) buffering amp or spare switched relay if I want to inject the audio while the radio source is still running.


It wouldn't hurt to go in either the phono or tape inputs just to boost the level and fidelity.
The solution continues.....


I was down due to issues with this, no pain just temporary blindness and much worse:

[David Hess]

I gave up coffee a couple years ago because of migraines and scintillating scotomas.

[Quarlo Klobrigney]

No more caffeine since I was in my early 30's. Every time a delivery or a customer came through, let's have a cup of coffee.
I hope yours was not the painful type.

Stress. If only you knew what I was put through in the last year., none of it having anything to do with something wrong with me.
A real lawyer could be rich. Alas, they are too lazy to take a long case, what with chasing ambuli and quick money.

Regarding the preamp, I have very small metal boxes that I could put 3 DIP-8 op-amps and a few passives in if I needed to, but a single dual will do.
Stick the lot of it in the shielded box with unregulated 12VDC. The input and output wires, (I only need mono) and see what the phono or tape input levels are like.

As you all can see, I'm slightly broken for a bit as I slowly get mended.
Thanks all for the suggestions and input above they are appreciated.

Next week will determine folly or success of the experiment.

[David Hess]

No more caffeine since I was in my early 30's. Every time a delivery or a customer came through, let's have a cup of coffee.

The problem for me was coffee, and not caffeine.  Coffee includes alkaloids other than caffeine.  At this point if I have 1/2 cup of weak coffee, then I will have a migraine within 24 hours.
 

I hope yours was not the painful type.

For decades they were only the painful type, but at the end I got more scintillating scotomas and they were more often accompanied with no pain, but all ended with euphoria which is a more definitive symptom of a migraine headache.  Migraines are not just extra painful headaches.