SOLVED
Hi all,
I know this is an old subject and I've successfully done this before for line level using both passive (resistors only) and active (op amp) circuits. But I'm having a problem - or a mind-block - getting it to work now on speaker level signals.
I'm using this module - https://www.amazon.co.uk/ZHITING-Bluetooth-Amplifier-Digital-Channel/dp/B088NFHMND/ref=sr_1_1?crid=3GA2N0JU85EEL&keywords=B088NFHMND&qid=1641051237&sprefix=b088nfhmnd,aps,87&sr=8-1
I need the amp because it's going into a valve radio phono input, so effectively it's acting as a pre-amp.
It works fine with just one channel. Volume levels are great and easily matched to the FM radio volume.
But I'm really struggling with summing the two stereo channels to mono. I seemed to have it working the other day but after spending some time working on something else on the radio, now it's not.
I'm doing it passively with resistors only (one each for each stereo signal, and one connecting the summed signal to the combined ground levels) but wondering if the are wrong or I'm just doing something stupid.
If I connect both speaker grounds together it causes the module to reboot.
Can anyone please help with exactly how I should be doing this please so the two stereo signals are summed to mono and equal in volume.
Thanks!
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Maybe answering my own question, it seems this is not a good idea and requires low value large wattage resistors? (I've read 6 ohm 10 watt per channel may work).
I've ditched the module I mentioned and have gone back to one I built myself a couple of years ago (takes 9v plus, goes down to 5v to power a chromecast audio, which is connected to an active summing circuit which boosts the signal enough to go to the phono input. It's not as loud as the circuit in the first post (ie, it's quieter than the FM radio level if I switch between them) but not massively.
I only changed because I'd developed a ground hum I couldn't get rid of and decided to try this circuit instead. Still had the ground hum. Fixed it on the radio itself, and just realised I might as well go back to my own circuit that already sums to mono.
I'd still be interested about the original question though, if anyone can confirm the low value high watt resistor solution. Thanks
I've ditched the module I mentioned and have gone back to one I built myself a couple of years ago (takes 9v plus, goes down to 5v to power a chromecast audio, which is connected to an active summing circuit which boosts the signal enough to go to the phono input. It's not as loud as the circuit in the first post (ie, it's quieter than the FM radio level if I switch between them) but not massively.
I only changed because I'd developed a ground hum I couldn't get rid of and decided to try this circuit instead. Still had the ground hum. Fixed it on the radio itself, and just realised I might as well go back to my own circuit that already sums to mono.
I'd still be interested about the original question though, if anyone can confirm the low value high watt resistor solution. Thanks
You bought a one-hundred Watt amplifier to drive micro-Watts into the Phono input of a radio?? Might be over-kill?
Further: those speaker outputs are not grounded. All the "obvious" connections will lose half the signal or short-out the amplifier. (Which would kill it, but the makers got tired of users doing that and apparently added a reboot function.)
And I don't see any need for low-Ohms high-Watts to make a mono line out.
This trick connection to only the "+" terminals of the L and R, plus a Gnd stolen from the AUX jack, might work fine. If too loud, lower the 100Ω resistor to 47 or 33Ω (conversely raise to 220Ω for "more", but that seems unlikely.)
The most obvious place to tap "Gnd" is the "shell" terminal of the AUX jack. Traditionally this was the contact closer to the panel; however iPhone and others have other schemes. The Gnd will likely land on a trace which runs all over the PCB. Carefully tack-solder a fine wire. I have assumed a "valve radio phono input" is screw terminal (also Google was being stupid about phono-jack images), but do whatever connection suits the situation.
Further: those speaker outputs are not grounded. All the "obvious" connections will lose half the signal or short-out the amplifier. (Which would kill it, but the makers got tired of users doing that and apparently added a reboot function.)
And I don't see any need for low-Ohms high-Watts to make a mono line out.
This trick connection to only the "+" terminals of the L and R, plus a Gnd stolen from the AUX jack, might work fine. If too loud, lower the 100Ω resistor to 47 or 33Ω (conversely raise to 220Ω for "more", but that seems unlikely.)
The most obvious place to tap "Gnd" is the "shell" terminal of the AUX jack. Traditionally this was the contact closer to the panel; however iPhone and others have other schemes. The Gnd will likely land on a trace which runs all over the PCB. Carefully tack-solder a fine wire. I have assumed a "valve radio phono input" is screw terminal (also Google was being stupid about phono-jack images), but do whatever connection suits the situation.
I presume Your amp is in BTL configuration and so it doesn't have a ground at speaker outputs. I would look at home theatre high level
sub input schematics.
Why on earth would you do this? You have a line-level signal, just sum that before your valve amp. Don't use a power amplifier!
Note also that a phono input probably has an RIAA filter on it, which will NOT give you the sound that you want - it is expecting to see a signal from a phono cartridge.
If you must sum the output of a bridged amplifier, you will want FOUR resistors, one for each terminal. Make the resistors about 10 ohms each. Also the signal level coming out of the power amp will be about 30dB too high, because of its internal gain. The noise performance will be abysmal.
Note also that a phono input probably has an RIAA filter on it, which will NOT give you the sound that you want - it is expecting to see a signal from a phono cartridge.
If you must sum the output of a bridged amplifier, you will want FOUR resistors, one for each terminal. Make the resistors about 10 ohms each. Also the signal level coming out of the power amp will be about 30dB too high, because of its internal gain. The noise performance will be abysmal.
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Sorry, where is the line level signal on the bluetooth circuit? If I was feeding in from the 3.5mm input, sure, but I am using the bluetooth.
The radio was made pre-RIAA (actually the year it was ratified but it doesn't have it).
This amp actually provides the right level needed to match the FM volume when it is set to 50 or 60% volume. I can assure you the sound is far from abysmal and is, in fact, great.
But as I've explained, I made a circuit for this myself 2 years ago that I am now using again. I'm using a small enclosed switching power supply to take the 230v that is going in to the radio to tap off 12v that goes to my circuit to power the op-amp summing which boosts the signal enough to make it usable (it's still lower than the in-built radio though, but it's acceptable and in reality I won't be switching to FM much). And then I step down the 12v to 5v with a MP1584EN to power a Google Chromecast Audio. The 3.5mm output ofwhich is plugged in to my circuit.
It works perfectly. The only issue was at some point, after moving it during an office makeover, it had developed a ground hum. It seemed to be related to my circuit so I tried this one from Amazon due to time constraints. But as that also had the ground hum (except when powered by battery), I managed to find the fault in the radio itself, and also fix two (small) issues I had done wrong when I restored/recapped the radio two years ago.
A valve radio typically would have a high impedance (because it's valve) input for a crystal pickup.
Say 100mV & the eq is inherent in the piezo cartridge being connected to the high impedance input.
This makes the input suitable as an aux input with some attenuation for higher level sources.
Say 100mV & the eq is inherent in the piezo cartridge being connected to the high impedance input.
This makes the input suitable as an aux input with some attenuation for higher level sources.
If you want bluetooth, just buy a bluetooth module without a power amplifier; they're about $5.
You really don't want extra power amplifiers present in your line-level stuff, especially that one because it is Class D.
You really don't want extra power amplifiers present in your line-level stuff, especially that one because it is Class D.
It's not line level. It does require pre-amplification. I've now gone back to my own circuit with does this as explained in other replies. Thank you.