Transform Low to Hi impedance at audio frequencies - Page 1
Forum section: electronics / projects

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 06:21:43 am
content: 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.

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2023, 09:01:11 am
content: 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 10:02:30 am
content: 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.

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2023, 10:27:25 am
content: 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.

author: strawberry
author_rank: Super Contributor
timestamp: September 23, 2023, 01:37:17 pm
content: 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 .

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2023, 02:37:06 pm
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: September 23, 2023, 02:52:08 pm
content: Quote from: Quarlo Klobrigney on September 23, 2023, 06:21:43 amA 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.

author: themadhippy
author_rank: Super Contributor
timestamp: September 23, 2023, 03:51:13 pm
content: Quotesure no theres correction eq on the input?Quoteno old transistor radios in the junk box to rip a tranny from?

author: CaptDon
author_rank: Super Contributor
timestamp: September 23, 2023, 03:51:34 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 04:07:09 pm
content: Quote from: Quarlo Klobrigney on September 23, 2023, 10:02:30 amJust 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.

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2023, 04:16:13 pm
content: 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 entryhttps://en.wikipedia.org/wiki/RIAA_equalizationshows 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).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 23, 2023, 04:43:32 pm
content: No idea what you're doing or why, but look up "common base" transistor amplifiers.

author: coppercone2
author_rank: Super Contributor
timestamp: September 23, 2023, 04:44:48 pm
content: well you can use a transformerotherwise 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.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 23, 2023, 05:05:16 pm
content: Quote from: Quarlo Klobrigney on September 23, 2023, 10:02:30 amIt'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.

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2023, 05:18:34 pm
content: 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 07:45:53 pm
content: 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|GNDIt'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.

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2023, 07:51:12 pm
content: "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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 08:00:31 pm
content: Quote from: Quarlo Klobrigney on September 23, 2023, 07:45:53 pmWhat exact make and model of tube gear are you using?

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 08:04:24 pm
content: Quote from: TimFox on September 23, 2023, 07:51:12 pmIt 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 08:12:26 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2023, 08:17:42 pm
content: 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.Anyhigh-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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 08:21:24 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2023, 08:47:21 pm
content: 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, tryhttps://www.radiomuseum.org/dsp_guest_schema_register-ng.cfm?schaltplan_id=136706Looking 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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 08:49:45 pm
content: Quote from: Quarlo Klobrigney on September 23, 2023, 08:12:26 pmIf you could state the exact model we might be able to look up the input schematics.QuoteIt'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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 09:15:58 pm
content: Quote from: TimFox on September 23, 2023, 08:47:21 pmLooks 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 09:20:18 pm
content: 3035https://nvhrbiblio.nl/schema/Grundig_3035WF.pdfEither 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 10:11:37 pm
content: 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 10:17:52 pm
content: TA = Phono in, TB Tape in.Both look like 1MΩ loads.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 10:25:28 pm
content: How does the radio sound?

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 10:49:42 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2023, 11:16:14 pm
content: OK then, can you measure the output of your BlueTooth module with a scope?

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 23, 2023, 11:25:44 pm
content: Can do, and the resistance of each channel.....Tomorrow as I get too tired too early now.

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2023, 09:17:24 am
content: 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 25, 2023, 12:24:26 pm
content: Next......

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 25, 2023, 01:02:50 pm
content: 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 onTodayat 04:17:24

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2023, 02:39:16 pm
content: Quote from: David Hess on September 25, 2023, 09:17:24 amA 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.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 26, 2023, 10:01:11 am
content: 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:

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2023, 03:34:50 pm
content: I gave up coffee a couple years ago because of migraines and scintillating scotomas.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 26, 2023, 06:22:23 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2023, 07:40:38 pm
content: Quote from: Quarlo Klobrigney on September 26, 2023, 06:22:23 pmThe 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.QuoteFor 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.