Stereo mixer amplifier

Greetings,

An article about my stereo mixer-amplifier project is now available.

formatting link

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
 Click to see the full signature
Reply to
Don Kuenz, KB7RPU
Loading thread data ...

The power supplies look strange to me. Why bias up the + inputs of U2 and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity and noise.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
jlarkin

And if there is a split rail already available why are the op-amps non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be used, one for each input, and the outputs of the transformer summed independently without signal loss but other than that it and the 50k attenutor just add noise. if they just used one transformer and a relay to switch the inputs you could use a passive attenuator of lower value, plenty of drive available from the line level signal and the TDA amp has plenty of gain.

Reply to
bitrex

At the very least if OP got rid of the AC coupling caps C3 and C10 and DC coupled the input stage the total attenuator value could be brought down to like 1k instead of 50k. There's already a 100uF DC blocking cap in series with the input like to the TDA like the datasheet says so why is 0.22uF C11 in series with that? and those other resistors. if the attenuator were lower value you could make it op amp biased to mid-point

-> attenuator -> DC blocking cap -> TDA input, that's it

Reply to
bitrex

If you used a relay to select an input it wouldn't be a mixer. The Opamps a re used as summing amplifiers to compensate for the resistive losses for ea ch input. That allows higher value potentiometers, and near zero interactio n between sources. Have you ever examined the design of a commercial audio console?

Split power supplies and what you consider as wasted parts are used to p roduce the required flatness and isolation along with the lowest overall sy stem noise. Even early tube mixing boards used transformer inputs, and the fades had to provide at least 80dB of attenuation at their lowest settings. Find some manuals and study them. The crap circuits used in guitar amps ar e useless for real audio processing. You can do balanced inputs or outputs with just opamps, but transformers eliminate ground loops which induce hum. A capacitor between a transformer and an active circuit prevent it from af fecting the biasing of the input or output stages.

Some of the first PA systems I worked on, I've despised unbalanced input s and passive mixers. Cheap crystal microphones, on long single conductor s hielded cables that picked up radio stations, and police cars were common i n the '50s and '60s.

I have worked with many broadcast and studio consoles, from Mono, eight input tube based Gates from the '40s to state of the art 40 input stereo co nsoles with six output busses. They all used transformer inputs and outputs , which allowed balanced or unbalanced inputs. Early equipment as 600/150 o hm, selected by using half or all of a winding at the connections.

All inputs were line or mic level. Turntables used external magnetic phono preamps.

At one very remote site, I had to use a spare mag phono preamp to bring the old WWII vintage telephone circuit up to an acceptable level. There was over 30dB of loss in the last mile of old cable, but the telephone people couldn't wrap their minds around anything without a dial and a bell.

Reply to
Michael Terrell

-----------------------------------------

Don Kuenz, KB7RPU

** The OP thinks he has to bias the coupling electros.

** See above.

** The OP is a radio ham.

BTW:

The input transformers used are of telephone quality.

The OP is a radio ham.

.... Phil

Reply to
Phil Allison

Ok right you are. The "mixer" part seemed to fade from my mind...

It's just there for a single TL0x dual op amp it has 100dB of PSU rejection. The TEA amp is single supply. A BU634P to make a virtual ground for 1 dual op amp seems like over-kill any way you cut it...

Ok but DC blocking caps on the mixing stage here are superfluous. You could provide the mid-point bias thru a buffered +6 reference to the tranny center taps and eliminate the caps and the split PSU entirely. maybe DC servo them with respect to the op-amp output to keep it at the mid-point.

Once the blocking cap on the op amp output is gone you can bring the attenuator impedance down. There's no good reason to use such a high-value attenuator after a low-impedance buffered output it just adds noise.

Reply to
bitrex

y
,

as

ps are used as summing amplifiers to compensate for the resistive losses fo r each input. That allows higher value potentiometers, and near zero intera ction between sources. Have you ever examined the design of a commercial au dio console?

to produce the required flatness and isolation along with the lowest overa ll system noise. Even early tube mixing boards used transformer inputs, and the fades had to provide at least 80dB of attenuation at their lowest sett ings. Find some manuals and study them. The crap circuits used in guitar am ps are useless for real audio processing. You can do balanced inputs or out puts with just opamps, but transformers eliminate ground loops which induce hum. A capacitor between a transformer and an active circuit prevent it fr om affecting the biasing of the input or output stages.

inputs and passive mixers. Cheap crystal microphones, on long single conduc tor shielded cables that picked up radio stations, and police cars were com mon in the '50s and '60s.

ight input tube based Gates from the '40s to state of the art 40 input ster eo consoles with six output busses. They all used transformer inputs and ou tputs, which allowed balanced or unbalanced inputs. Early equipment as 600/

150 ohm, selected by using half or all of a winding at the connections.

ono preamps.

ring the old WWII vintage telephone circuit up to an acceptable level. Ther e was over 30dB of loss in the last mile of old cable, but the telephone pe ople couldn't wrap their minds around anything without a dial and a bell.

It isn't studio or broadcast oriented. It simply allows him to use one set of speakers for four devices at once. Since he is an Amateur radio operator , he has to consider RF ingression. Design something better and post it, if you can.

Reply to
Michael Terrell

If you're gonna use a jellybean 20 cent TL-series dual op amp as your mixer not much point worrying about having a super low-noise audiophile-quality split power supply here for 'em, y'know

Reply to
bitrex

If he's worried about RF ingression there should probably be something in the circuit that would help with that.

Reply to
bitrex

------------------

** TL081s used like this ( unity gain inverting ) have vanishing THD and very low noise. They have good PSSRs too, at supply frequencies.

Silly to piss on them, like some arrogant smart ass.

.... Phil

Reply to
Phil Allison

ay

e,

has

d parts are

It is a monitor amp, nothing more. It does what he wants. It's you that wan ts super low-noise audiophile-quality. You've never had to deal with a volt or more of RF on the shield of a circuit like this. I've had to completely recable a TV studio to remove RF from the station's Video feed to their TV transmitter. The entire station had grounding problems. You couldn't pick up any phone in the building and not hear the 980KHz AM transmitter that wa s about 25 feet from the back of the main building. The upgrades included d riving almost 80 feet of ground rod through permafrost, until I reached sui tible soil conductivity. I also had to cut the shield at all of the balance d inputs because of the high RF field. Those coupling caps help limit the a udio range to the desired bandwidth, as well. Those power amp modules can't handle RF without becoming unstable or frying themselves.

Reply to
Michael Terrell

Not trying to piss on them, just that a single TL081 doesn't need a rail-splitter IC that costs nearly $12 in singles to do its job perfectly well.

It's a good value for the money in this application. I mean, before the OP added the above.

Reply to
bitrex

-----------------------

** Had to deal with that a few times in the world of *pro audio*.
  1. Roland 31 band graphic picking up parked taxis near a venue, identified as the culprit in a PA system since the hard bypass button stoped the drive r's voices.
  2. Marshall valve bass head piking up 2-way VHF walkie-talkies in a club us ed by security.
  3. Steel string acoustic ( Ovation ) guitar could not be used with a plug i n VHF radio Tx cos it made nasty clicking noises every time the player fing ered a sting against a fret.

All the above were fixed by adding 10 or 100nF ceramics from input connecto r grounds direct to the metal chassis in each case.

But one job that was beyond help was a recently constructed mid-size record ing studio on the 12 floor of an inner city building. It was possessed with buzzing noises ( TV frame) from a VHF tower on the top of a nearby buildin g. Most of the installed gear and worse, any gear clients brought in, were badly affected - made recoding impossible.

I immediately knew the answer - full, copper mesh screening. But all the so und proofing and acoustic treatment had been done before the problem was di scovered.

Oh dear ......

.... Phil

Reply to
Phil Allison

If I wanted to sell to audiophiles I'd be adding $12 buffer ICs, not suggesting to remove them!

Reply to
bitrex

--------------

** Those cheap and nasty audio transformers on each inputlikely make dandy RF filters ...

.... Phil

Reply to
Phil Allison

This device suits my needs quite well. During normal operation it inputs background music from a PC as well as a radio typically tuned to the Maritime Network during the day and a lively 40 or 80 meter frequency at night. Sometimes the output from a video processing server is added to the mix. It can take a while to create an mpeg file from an edited video stream. After my video editing software completes its task it plays the clip from the beginning. Then the soundtrack becomes audible over my mixer amplifier to alert me to the completion of the task. The fourth input's there to accommodate experiments.

The BUF634P discussion arose the last time my circuit was posted a few months ago. And so my webpage touches on the BUF634P in a couple of places.

formatting link

presents a nice overview of the tradeoffs involved. During construction a resistor and a zenier diode were substituted for the BUF634P and the zenier diode solution degraded the sound. Although a TLE2426 rail splitter may provide an adequate solution this device functions perfectly well "as is." It's time for me to abandon this project and move on to the other projects in my backlog. As noted on my webpage, a project's never finished, it is only abandoned.

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
 Click to see the full signature
Reply to
Don Kuenz, KB7RPU

I thought the transformer choice might be perhaps because the mixer was for listening to four stations simultaneously. Like "picture in picture" on the TV, but for hams. But inputs are stereo.

I'm 40 and I can only hear out to 12kHz. Perhaps at 65-70 one's auditory bandwidth is such that one can start saving a few bucks on transformers. "That music the kids listen to today is all just racket!"

Reply to
bitrex

be

k

elay

lue,

p has

e

ted parts are

io

r
g

ants super low-noise audiophile-quality. You've never had to deal with a vo lt or more of RF on the shield of a circuit like this. I've had to complete ly recable a TV studio to remove RF from the station's Video feed to their TV transmitter. The entire station had grounding problems. You couldn't pic k up any phone in the building and not hear the 980KHz AM transmitter that was about 25 feet from the back of the main building. The upgrades included driving almost 80 feet of ground rod through permafrost, until I reached s uitible soil conductivity. I also had to cut the shield at all of the balan ced inputs because of the high RF field. Those coupling caps help limit the audio range to the desired bandwidth, as well. Those power amp modules can 't handle RF without becoming unstable or frying themselves.

Back in the 1960s through the 1990s, you couldn't pick up a phone anywhere in northern New Jersey without hearing WABC 770KHz AM.

Reply to
rangerssuck

s

n be

ed

50k

relay

value,

amp has

ive

d

asted parts are

udio

,

tor

ing

wants super low-noise audiophile-quality. You've never had to deal with a volt or more of RF on the shield of a circuit like this. I've had to comple tely recable a TV studio to remove RF from the station's Video feed to thei r TV transmitter. The entire station had grounding problems. You couldn't p ick up any phone in the building and not hear the 980KHz AM transmitter tha t was about 25 feet from the back of the main building. The upgrades includ ed driving almost 80 feet of ground rod through permafrost, until I reached suitible soil conductivity. I also had to cut the shield at all of the bal anced inputs because of the high RF field. Those coupling caps help limit t he audio range to the desired bandwidth, as well. Those power amp modules c an't handle RF without becoming unstable or frying themselves.

e in northern New Jersey without hearing WABC 770KHz AM.

Likely caused by the same thing that I found. Too much old wiring, left fro m previous installs but still connected to the CO. Our building was fed wit h a 25 pair (and spare) lead jacketed cable and transformers for Phantom pa irs that were left from WW II when the building had been a Russian Pilot's mess and Barracks. I ran two 25 pair lines to two new Demarcs, then short r uns to the phones. Then I removed over a mile of three or four conductor st ation wire. One Demarc was at the pair of Teletype machines and offices. Th e second was in a rack in the Radio studio. The original was in the control room for the TV station. I still wish that I had taken those phantom coils from the wall before I left. They are collector items, these days.

Phone companies were really bad about leaving old wire in hope of reusing i t some day. The Military used Bell System Practices, and had followed suit.

Reply to
Michael Terrell

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.