Suitable audio transformer for balanced to unbalanced - Page 1
Forum section: electronics / beginners

author: UHF
author_rank: Contributor
timestamp: October 03, 2024, 03:22:34 am
content: Hello, I'm making a pair of small, powered desktop speakers to take the main balanced output from a PreSonus Studio 1810c audio interface. The single speaker will be an 8 ohm full range driver, driven by a "TPA3110 PBTL Mono Digital Power Amplifier Board" from Aliexpress, with 12V power from outside. I want to take the balanced output from the PreSonus and then send it through an audio transformer (passive) - balanced to unbalanced and then feed this into the TPA3110 PCB, again an Aliexpress transformer for cost reasons. The PreSonus main outs produce up to 2Vpp from GND to "hot". The idea is to just use 12V to power each speaker and set the volume from the PreSonus unit.Is this acceptable? What value transformer should I use, 10K:10K? I feel the output impedance from the PreSonus is high, but I've no idea what a typical value might be. Many thanks.

author: langwadt
author_rank: Super Contributor
timestamp: October 03, 2024, 08:20:10 am
content: why do you want a transformer?

author: UHF
author_rank: Contributor
timestamp: October 03, 2024, 09:29:51 am
content: Can I just connect GND and 'hot' to the amp input and forget about converting balanced -> unbalanced?

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 03, 2024, 09:49:18 am
content: Direct connection may work but with lots of noise and it´s definitely not the way it should be.I´d take an opamp:https://community.element14.com/technologies/test-and-measurement/b/blog/posts/opamp-basics-balanced-to-unbalanced-signal-with-difference-amplifier-699051511

author: langwadt
author_rank: Super Contributor
timestamp: October 03, 2024, 10:09:38 am
content: Quote from: Phil1977 on October 03, 2024, 09:49:18 amlots of noise from where? it a line level signal, presumably a short distance

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 03, 2024, 10:37:53 am
content: Noise e.g. from common mode currents that originate from the power supplies?Most devices with symmetrical outputs easily provide unsymmetrical signals if you connect GND and + from the symmetrical output to GND and signal of the unsymmetrical input. But why not make use of the the symmetrical transmission if it only costs one opamp and protects you from a lot of ground loop issues?For sure it´s worth a try to connect it directly. I just say I think it´s better to use an opamp instead of a transformer.PS: Sorry, I didn't see the OP has also directly asked about that. Yes, it´s possible to directly connect. But it´s more sensible to noise.

author: themadhippy
author_rank: Super Contributor
timestamp: October 03, 2024, 11:21:07 am
content: cheap skate method thats been used for years were isolation aint required is to tie the - (pin 3 on an xlr) to the ground(pin 1)

author: UHF
author_rank: Contributor
timestamp: October 03, 2024, 11:59:01 am
content: The cable from audio interface to speaker won't be more than a metre, but I'd like to do it properly. I'd prefer to stick to having 12V from a power adapter rather than an internal PSU in the speakers. Using an op-amp or line receiver to convert from balanced would require a bipolar supply I think.What transformer would be appropriate? Theres's a range on Aliexpress for audio/DI boxes with 600:600, 600:10K, 10K:10K etc.

author: ftg
author_rank: Frequent Contributor
timestamp: October 03, 2024, 12:58:46 pm
content: I have used 600:600 for some radio audio interfaces.But that is for just 300 - 3000 Hz or so.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2024, 01:06:26 pm
content: For full-audio range, at higher price, I have had good results with these small transformers from Lehle:https://www.tubesandmore.com/products/transformers_chokes?filters=2341a3027"HZ" goes from low impedance to high impedance"LZ" is 1:1 in impedanceClick on the individual listing to get full specs.

author: abeyer
author_rank: Frequent Contributor
timestamp: October 03, 2024, 03:31:53 pm
content: Quote from: UHF on October 03, 2024, 11:59:01 amWould just using or scavenging parts from a DI make sense vs trying to source from cheap aliex options yourself? Dunno where you are, but at least around me, I see nice pro grade DI boxes on the secondhand market for cheap all the time. I picked up a pretty nice two channel one for ~5 USD a few months ago.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 03, 2024, 04:03:58 pm
content: Quote from: UHF on October 03, 2024, 11:59:01 amI disagree.

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2024, 07:01:37 pm
content: TPA3110 does have differential inputs, but not brought out on those boards. You'd have to solder a wire to that cap. Or just be lazy and run it it single-ended.The amplifier is Class D and I would expect it to make so much EMI and only a top quality audio transformer is going to do something for that - which is none of the ones on Aliexpress. Those will just add colouration and distortion and no proper CM rejection. They haven't learned how to wind them properly, and commonly use power ferrites.

author: UHF
author_rank: Contributor
timestamp: October 03, 2024, 11:35:26 pm
content: Thanks for the replies. Any ideas about suitable transformer impedance for line levels? I think 600ohms is for mics usually?

author: BrianHG
author_rank: Super Contributor
timestamp: October 03, 2024, 11:47:44 pm
content: If you need quality, go here:https://www.lundahltransformers.com/pro-audio/This one isn't too bad at 300$:https://www.lundahltransformers.com/wp-content/uploads/datasheets/SIB15.pdfRemember that the speaker output load needs to be 8ohm or 4ohm to get a flat audio response as class D amps need that load to balance their output ripple filter.Also remember that if there is a speaker on the output, it doe not have a flat load, so the audio wont be flat of have perfect phase as if you have a class AB output amp with a high current or damping factor.Also, divide the speaker output into the Lundahl transformer with a resistor divider network so you never exceed the transformer's 3vrms, 8vp-p limit.

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 04, 2024, 08:38:16 am
content: I didn't know acceptable audio transformers are that expensiveAnother reason to use an opamp - bipolar supply is not mandatory.Or just use a reverse-DI-box.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 02:16:11 pm
content: For signal levels, the Lehle transformers I cited are much cheaper than the Lundahls.If you look at the specs for the Lehle, you see that the distortion is quite good, although it increases at low frequencies with higher voltage.(This is normal for a transformer, and is an important spec.)

author: BrianHG
author_rank: Super Contributor
timestamp: October 04, 2024, 03:11:18 pm
content: Quote from: Phil1977 on October 04, 2024, 08:38:16 amMaybe the OP should think about tapping the amp's input instead of its output....

author: themadhippy
author_rank: Super Contributor
timestamp: October 04, 2024, 03:36:16 pm
content: Quoteerm they are,Quotethe prosonus is a pc sound card not an amplifier

author: BrianHG
author_rank: Super Contributor
timestamp: October 04, 2024, 04:46:40 pm
content: Quote from: themadhippy on October 04, 2024, 03:36:16 pmNow I'm even more confused. When using the TPA3110 in mono mode, it is designed to receive a balanced input to operate properly. If the OP has a balanced source and is using an amp chip with a balanced input, why would he want to convert his source to unbalanced? The datasheet of the TPA3110 specifically illustrated a + signal driving the + side for the speaker and a separate - input to drive the - side of the speaker in mono mode.