Transform Low to Hi impedance at audio frequencies

[Quarlo Klobrigney]

I'm looking for any circuit ideas to transform a low to hi impedance at audio frequencies.
Everywhere there there are the reverse i.e. high to low.
I'm back from the dead for now.


Does anybody have a clue for me to accomplish this without any distortion or tonal alterations?


Transformers do not count as cost would unacceptable where junk box components could suffice.
For example a simple circuit with some passives, a single supply with a ubiquitous op-amp, transistor or FET.


If possible either positive or negative gain if needed.


Please your suggestions. Thanks.

[moffy]

You need to provide more detail about the application. Is this for signal level or power level? What impedances do you want to convert between and why? Do you want to scale the voltage as well as the impedance? If you want to go from low to high impedance just use a series resistor.

[Quarlo Klobrigney]

That would be around 100Ω to an input of 1MΩ.


The output comes from a Bluetooth module to a tube tape or phono input.


The way it is now the audio is thin even though the BT modules is plenty loud.


The input looks like a 100KΩ to ground as a load, and a 2MΩ series resistor to the amp circuit.


Only a slight mismatch!


Excuse me for what might seem to be an easy task. I should have included the specs.
I've been in hospital the first part of the year so all of my circuits are not firing well yet.

[moffy]

Increasing the output impedance won't increase the signal level. A phono input is normally quite sensitive. If you want to try amplification then just try a suitable gain op amp. How do you measure the loudness of the output of the BT module? Sorry to hear about your ill health, I can sympathise with how it affects the ability to think.

[strawberry]

old generation BT dont have quality DAC nor bandwidth . no worries about mismatched transmission lines here . line signal level is ~1V but phono is ~10mV . attenuator and following low noise buffer will do the job .

[TimFox]

Rule-of-thumb:  adding a resistor to a circuit that works without it will always reduce the signal/noise ratio.
This is true for either a resistor added in series with the circuit, or in parallel with the amplifier input.
Of course, this is a quantitative question.
Transformers can accomplish your transformation, but a good one can be large and expensive.
With a low output impedance from your source, a bipolar-input op amp can amplify the voltage without much degradation of SNR.
(If it were a high output impedance, a JFET-input would probably be better.)
Note that a CB (grounded base) amplifier will present a low impedance to the source, but it will degrade SNR since its input noise generators are close to those for the normal CE (grounded emitter) amplifier.

[Marco]

I'm looking for any circuit ideas to transform a low to hi impedance at audio frequencies.

A resistor.

But I suspect what you should be looking for is insight for when impedance matching is relevant. A transformer increases voltage while increasing impedance, so you can get a SNR gain. Merely matching the impedance with any other circuit doesn't necessarily have to do the same, the resistor will match the impedance just fine and still decrease SNR.

There might be some switched inductor shenanigans you can pull to create a compact equivalent of a transformer, but since this is for audio the audience will recoil in horror.

[themadhippy]

The output comes from a Bluetooth module to a tube tape or phono input.


The way it is now the audio is thin even though the BT modules is plenty loud.

sure no theres correction eq on the input?

Transformers do not count as cost would unacceptable where junk box components could suffice.

no old transistor radios in the junk box to rip a tranny from?

[CaptDon]

First off, a low impedance source driving a high impedance input isn't a problem. No matching needed. Second off, Your phono input may have RIAA equalization and isn't flat. Another problem may be a reversed phase on the input cancelling all low frequency content. Phono inputs can be for crystal cartridges and expect or tolerate input voltages as much as .5vpp without clipping distortion where as dynamic cartridge inputs may only tolerate 100mvpp without clipping. I seriously don't think impedance mismatch is the root of your problem. You could try loading the bluetooth output with 100 ohm resistors for the left and right channels. Perhaps the bluetooth device doesn't like being unloaded.

[bdunham7]

That would be around 100Ω to an input of 1MΩ.


The output comes from a Bluetooth module to a tube tape or phono input.


The way it is now the audio is thin even though the BT modules is plenty loud.

Just tell us exactly what the tube amplifier or console is, someone may be familiar with it.  The impedance 'mismatch' shouldn't be a problem, but other characteristics of the input may be.  Try putting a 10K resistor across the input and see if there are any changes. 

[TimFox]

In a tube amplifier, there may be "tape" and "phono" inputs.
These are high-gain (low-level input) with a serious "equalization" non-flat frequency response ("NAB" and "RIAA" for tape and phono, respectively).
If you look at the frequency response of a RIAA amplifier, it is a strong low-pass filter (lower gain at high frequencies).

The red curve ("playback") at the top of the Wikipedia entry  https://en.wikipedia.org/wiki/RIAA_equalization  shows that the response at 20 kHz is 20 dB down from 1 kHz and 40 dB down from 20 Hz.
This would definitely give a "thin" sound from a source that did not use the inverse of that frequency response (blue "recording" curve in Wikipedia).

[Conrad Hoffman]

No idea what you're doing or why, but look up "common base" transistor amplifiers.

[coppercone2]

well you can use a transformer

otherwise you can use a amplifier to boost the signal voltage and then put a resistor to limit current?

I guess this would be useful if a device by design was counting on the output being loaded for some reason? so basically the opposite of signal integrity.

[ejeffrey]

That would be around 100Ω to an input of 1MΩ.


The output comes from a Bluetooth module to a tube tape or phono input.


The way it is now the audio is thin even though the BT modules is plenty loud.

It's normal and desirable in audio to have a low output impedance driving a high input impedance. Impedance matching of most audio circuits is not desirable.  It's possible you need gain or equalization.  While a transformer can provide voltage gain it's rarely the best way to do it.

Specifically if you are using a phono input that is actually designed for unamplified phonograph needle inputs that may have a RIAA gain curve?  If you feed normal audio sources to that it will be too loud but not sound right.

[TimFox]

The poster has connected to a "tape" or "phono" input on a vacuum-tube amp, which undoubtedly has an inappropriate frequency response (e.g., RIAA) for his purpose, producing an output with very low high-frequency content.
It was very rare on vacuum-tube amplifiers (except for guitar amps) to have a "flat" low-level (high-gain) input, which would have been labeled "mic" or similar for a microphone.
If his signal level is not high enough for the "aux" input (flat response, but higher input level/lower gain), then an amplifier or transformer is required to drive the amplifier.

Also, for this application (low source impedance driving a high load impedance at audio frequencies) there is no advantage for a CB (grounded base) amplifier over the more popular CE (grounded emitter) amplifier.
The CE amplifier will have higher voltage gain, with approximately the same input noise voltage.

[Quarlo Klobrigney]

I had a bit of a nap while you folks bantered to and fro. I'm still recovering.

I don't have any transformers other than from a supplier and we know what they cost today.

The input is strictly a Euro one so whatever that is DIN or ??
The EQ response is unknown.

As I have stated the idea in schematic form is not found with Google.

I had even foolishly asked Chat GPT v3.5, and every iteration resulted in gibberish in an ASCII schematic, circa 1993.
It had no idea including 80% of the results being a high to low circuit in its description.
It's last go was the very clear circuit as shown below.

Vin (Low Impedance)
    |
   C1
    |
    +-------- Out (High Impedance)
    |
    |
   R1
    |
    |
   ---
    -
    |
    |
   C2
    |
  GND
   
It's quite clear and one can even see how the op-amp is doing the transformation,
yes there's a TLO-71 or 81 in there.....

Please help me in finding a solution to this issue, I only want to listen to my programs and music via hollow-state fidelity and technology.

[TimFox]

"Unknown EQ response":  if the input is labeled "phono", it is almost certainly RIAA, which you don't want.
If the input is labeled "tape", it could be a high-gain input with "NAB" or similar equalization (designed to be driven from a tape head) or a low-gain input for the amplified output from a tape deck with flat frequency response (similar to the input for an external tuner or "AUX" input).
The "tuner" or "aux" input may not have enough gain for your source.
The circuit abbreviated with C1, C2, and R1 is a coupling network from a low-impedance source to a high-impedance load, that may have an equalization-useful frequency response.

[bdunham7]

Please help me in finding a solution to this issue, I only want to listen to my programs and music via hollow-state fidelity and technology.

What exact make and model of tube gear are you using?

[bdunham7]

"Unknown EQ response":  if the input is labeled "phono", it is almost certainly RIAA, which you don't want.

It depends on the era and what was connected to it.  Older console systems that are intended to be used with a ceramic phono cartridge may have radically different EQs or none at all.  My only tubular audio relic has a mono phono input with no equalization except a high-pass filter that rolls off most of the low bass.  It also is pretty high level, you can connect a phone or computer to it directly although that would have been a bad idea before I modified it to not be live-chassis.

[Quarlo Klobrigney]

A Grundig from 1957.
I can't even get the connect tones to come through. Windows USB connect like tones.
They sound like very low whispers.
I reboot the BT module on each resistor capacitor iteration expecting a different result to no avail.

I must also say from either the phono or tape input the buzz from a probe and the ambient AC is very loud.
I, being of high impedance, overwhelm the input, where the BT module does not.
It's an impedance issue.

[TimFox]

Piezo (ceramic or crystal) phono cartridges respond to needle displacement, while magnetic cartridges respond to needle velocity.
The electronic equalization curves are therefore very different, and cheap gear omitted any real equalization from the piezo-input amplifiers.
Also, the output voltage from a piezo cartridge is much higher than that from a magnetic cartridge.
Any high-impedance input will buzz loudly when you touch it, or don't connect it to ground through a low impedance.
Your low-impedance BT source essentially shorts out the high-Z input and removes the hum/buzz.
Your real problem (assuming the Grundig is in good operating condition) is not an impedance issue.

[Quarlo Klobrigney]

It is an impedance issue and the Grundig and the rest of the fleet are in perfect restored condition.
The Grundig being a "portable" table radio as opposed to the rest of the furniture.
A good "let's have a lie down" radio in the bedroom.

Which takes me to my original post above.
A low to high impedance amp or buffer to make it so, with my brain after being in and out of hospital from the first of the year, is incapable of performing such "advanced" tasks right now.

[TimFox]

I disagree about impedance transformation, as stated above.
Does the back panel of your Grundig have an input labeled "PU or Grundig tape recorder"?
"Retroradiofarm.com" will add Bluetooth to a working Grundig for $99.00.)
If you need a schematic, try  https://www.radiomuseum.org/dsp_guest_schema_register-ng.cfm?schaltplan_id=136706
Looking at the low-res thumbnail for the Grundig 97 at that site, there is nothing fancy in the audio chain (triode and one pentode), so I believe that bdunham7's post is correct that it is for a piezo cartridge.
However, the unit is an "AC/DC" set, without power transformer, so you have to be careful about connecting to externally grounded stuff.

[bdunham7]

A Grundig from 1957

If you could state the exact model we might be able to look up the input schematics.

It's an impedance issue.

It's a signal level issue.  A transformer might simultaneously raise both, but the only one you need is the signal level.  A 10K or 1M resistor in series will raise the output impedance of your BT module, but that won't help you.

If your Grundig tabletop is set up like my Zenith was, you might try reversing the power plug to see what effect it has on the buzzing and hum when you touch it.

[bdunham7]

However, the unit is an "AC/DC" set, without power transformer, so you have to be careful about connecting to externally grounded stuff.

Looks like it has a transformer, but I'm not sure it is the correct model as I don't see where the tape/phono inputs would be.  My Zenith was AC/DC, but I added a transformer, rectifier and regulator.