Passive summer/mixer to combine a differential speaker-level signal and a single - Page 1
Forum section: electronics / general

author: kubark42
author_rank: Contributor
timestamp: December 07, 2023, 01:47:59 am
content: For the input to a mono amplifier, I would like to mix together a differential signal coming from aPAM8302(differential amplifier, 2.5W to 4 Ohm load) with a single-ended line level signal. The differential signal is a continuous series of constant-volue beeps and boops, and the the single-ended is an occasional computer voice. Audio fidelity is almost completely irrelevant, and the volume can be controlled independently for each device.For (lack of) power reasons, the mixer needs to be passive. However, the result shouldn't be heavy (weight is more critical than audio quality).Is there a schematic for such a beast?Is it as easy as puttinghttps://www.alliedcomponents.com/storage/transformers/pdfs/at4365.pdfin the circuit and hooking up the two outputs to two windings and the input to the third winding?

author: themadhippy
author_rank: Super Contributor
timestamp: December 07, 2023, 03:08:55 am
content: QuoteThe output of the amp will more than likely be to hot to feed into a further amp,id use a couple of resistors to bring the level down to something a bit more usable ,feed that into a transformer and the output into a simple passive mixer with the line level source

author: kubark42
author_rank: Contributor
timestamp: December 07, 2023, 01:22:53 pm
content: Thanks for the answer. If I've understood your suggestion correctly, it's something like the attached schematic, right? If so, would the resistor be necessary, could I not accomplish the same thing with a step-down transformer?

author: kubark42
author_rank: Contributor
timestamp: December 07, 2023, 01:28:06 pm
content: Or by "couple of resistors", did you mean a voltage divider like in the below? If I have a voltage divider, do I then need the transformer if the stereo-level input is constant volume?

author: themadhippy
author_rank: Super Contributor
timestamp: December 07, 2023, 02:32:04 pm
content: QuoteyepQuoteyes,it avoids the differential output of the amp being grounded.

author: kubark42
author_rank: Contributor
timestamp: December 07, 2023, 02:34:50 pm
content: Awesome, thanks! Last question: if the purpose of the trafo is DC-blocking, could I do the same thing with a cap, as in the attached circuit?

author: themadhippy
author_rank: Super Contributor
timestamp: December 07, 2023, 03:43:08 pm
content: No,the transformer is to avoid the differential signal being shorted to ground.

author: kubark42
author_rank: Contributor
timestamp: December 09, 2023, 12:47:13 pm
content: Is this what you meant, for a range of 300-3kHz?https://falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmB0BskBMAWdn0B2SQwgTgmSxCQUhoFMBaMMAKADcV0AObvhIXT8o4cJATCI9GdAScRYISBxkpy+hDCQK0aljmjZ8gOYjkvETjDCZbAE41lFvoQTUXo7XDYB3EG7USsKBIFiSUH4B7iKh4bZsAPLR+hE8amERdma8fFi4ILlhhNR2AEqKzpaspaI4mkZQcmwAKoUFwYU8eQWycFpN6AhkkOhkCKpqyMiQSk3wkMg4WDxDCFToy+FgOE1k+1GeuPSenZAOXXydoWfiPo6SmsqPYfV1F6rqwi+3OIeWxxomFedn8P2U6A6GjYOQK+V2kN2WBKRjYAHtCiA+PR6mQ8oYYNJwGxkHxkCAAGJErQLeAgFggMoMADOAEtmQAXACGADsAMYMEnYymUFBwMXihkU+wMACOAFcGPyAJ5CsRUlAobRa8UQAA65yAAAlso, a blocking cap with a pull-down seems to work across the same range of 300-3kHz:https://falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcAWaAmAzAgbBjB2BSfZBNBADhAQ2usmoFMBaMMAKADcQNsreqCfNh58o4cGAISGs6Ai6iqYYSGQBOESpEMIbEtHUg088XIUBzJdYFqwOqOwBO1VbfxoGt3RnLsA7iAeuqrBxjiOgWHunuE67ADyQbHkIhSacWbsVtiQVGjIDLn5HlkAStbalWBoZmq6dTAKLsXgobFVXmjYCoEaWqpkyqqQ7ADG1GjDIq2dULDwkGDIWrC8K-gYYJBo6mSbSDBwHC15bTNnBbLgi3DsAPYgWg6F6oLzELrsGPwgAGKfegMTxwECsEBlRgAZwAllCAC4AQwAdmNGN8qEYAeIiAxCqDwZDYQiUWiMRJsbQtl5tgxwX8nIwAI4AV0YqIAnuSGJSeKCMPBjCAADqjIANaively, I feel like the cap is the better solution because there's very little power going through it, whereas the transformer solution runs a lot of power. Is perhaps my circuit wrong, or am I choosing the wrong values?

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 09, 2023, 03:17:54 pm
content: Quote from: kubark42 on December 07, 2023, 01:47:59 amJust use one end of the differential signal, as you say quality isn't important. Actually, the best would be to tap into the input of the existing amplifier if such an option would be available.

author: themadhippy
author_rank: Super Contributor
timestamp: December 09, 2023, 07:49:22 pm
content: Quoteits nothing to do with sound quality,its about not destroying the output stage of the pam 8302 .If you look at the functional diagram you see each of the outputs (vo+ and vo-) can be pulled to either positive or ground,if you ground output vo2 ,whats going to happen when its being pulled positive? even if it dont get upset ,at the same time v+ will also be at ground and the resulting signal will be ?

author: BrianHG
author_rank: Super Contributor
timestamp: December 09, 2023, 08:48:02 pm
content: (This should have been posted in the 'Beginners' section as it may have gotten more views)Is no-one here going to correct this guy's erroneous schematic use of a transformer to convert a balanced floating source to a unbalanced output for mixing AND solve the unfiltered class D output of the PAM8302 which may be ok to directly feed a small speaker but will make havoc driving a line level input's with those high frequency square waves. (Ignore this second part if the user's PAM8302's output already goes through a choke inductor.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 09, 2023, 09:04:58 pm
content: Quote from: themadhippy on December 09, 2023, 07:49:22 pmLeave the other end unconnected.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 10, 2023, 12:21:10 pm
content: Quote from: kubark42 on December 07, 2023, 01:47:59 amThe PAM8302 is a filterless class-D amplifier so definitely cannot be mixed as an audio signal as its bandwidth is well beyond audio.You could mix the signal going into the PAM8302 with the other signal though.