Bit of advice - preamp

[Mosaic]

This slightly confusing to me, have a look here:
**broken link removed**

It states it needs 12VAC-0-12VAC , 3 pin supply.
Now is that a 12VAC supply with a centre tap ground or a 24VAC supply with a centre tap ground?

 

[ronsimpson]

The NE5534 works from +/-3V to +/-20V. (don't use it at +/-20, or 40V, that is the max)
Ebay said 12VAC-0-12VAC or 15VAC-0-15VAC

 

[JimB]

It states it needs 12VAC-0-12VAC , 3 pin supply.

Yes that means that it needs a 24v centre tapped supply.

JimB

 

[audioguru]

12VAC-0-12VAC when rectified and filtered produces plus and minus 15VDC.

 

[Mosaic]

Thx guys, now I need to connect this preamp into a class D power amp that uses a 19VDC supply. I am concerned about the grounding reference. Won't the (+/- 12V) supplied NE5532 signals outputs be swinging -ve like true ac? What would happen when I tie the preamp 'centre gnd' to the Power amp zero V gnd? I am feeding the preamp with a 10VAC-0-10VAC. CT 1A transformer
This is the power amp.
**broken link removed**

 

[audioguru]

The e-bay power supply for the class-D amp says Output Power: 72W MAX.
The datasheet for the TDA7492 amp IC shows that with a 19V supply and an 8 ohm speaker it begins clipping at about 12W and has 1% distortion at 18W.
With a 19V supply and a 4 ohm speaker it begins clipping at about 17W and has 1% distortion at 29W.
FAR from 50W per channel!

The input of the power amp should have a coupling capacitor so that the signal from the preamp can swing negative and positive with no problems.

 

[Mosaic]

I did select an overrated amp for the app...I know that pushing for 50W will be mostly 'noise'. So no more than perhaps 25W then.
Do you think its reasonable to split the 19VDC supply to feed the preamp with virtual +9.5 0 -9.5V? . Then to tie line level signal to the power amp using the gnd ( which is the -9.5V in the preamp?). Using a coupling capacitor. The AF of interest is < 5Khz.

Active splitter:

 

[audioguru]

You did not show the schematic of your NE5532 preamp.
ANY audio opamp circuit with a split supply can be easily biased then use a single-polarity supply. Then its ground is the same ground as the power amp ground.

 

[Mosaic]

I don't have the NE5532 schematic as the Ebay preamp item comes assembled. I could try to decipher one channel using continuity checks as all parts are thru hole. Should I do that so you can advise me better? The pic is the preamp I have, it carries tone controls.



**broken link removed**

 

[audioguru]

The Chinese circuit has a horrible translation. It has NO specifications.
Try your voltage splitting circuit with it.

 

[Mosaic]

Thx, I'll give it a go shortly. I am amplifying an electret 'pc' microphone in a highish SPL environment. I'll probably dampen it with wadding if the SPL is too high. (inside the skirt of a tenor steel pan). The pickup works on the bench as fed by the +9v for the FET via a 10K.


What do you think of this approach though? I only need mono btw.
https://www.instructables.com/id/DIY-HiFi-Gainclone-Power-Amplifier-2x68w-Class-A/

Also , if you could suggest a 'decent' NE 5532 schematic (there are so many!) , that can be biased to work with these OTC poweramps or power amp kits I could build it if things become difficult with the chinese design.

 

[audioguru]

The Chinese "preamp" you linked is a tone controls circuit, I do not think it will amplify an electret microphone.
Here is my electret mic preamp circuit that works with a TL071 opamp or a NE5532 opamp with the same pin numbers. Use a 7809 voltage regulator with the input and output capacitors shown on its datasheet to reduce the supply to +9V.

The LM3886 is an excellent class-AB power amplifier. Make the circuit and pcb shown on its datasheet, not the Instructable from that little kid.

EDIT: Oh. Caribbean, steel drum? It will be very loud to the electret mic. Reduce the value of the gain trimpot in my preamp circuit.
There is a way to modify an electret mic so that its Jfet operates as a source follower which is good for loud sounds instead of as a low level common source amplifier.

 

[Mosaic]

Did u forget the link?

 

[Nigel Goodwin]

Did u forget the link?

Check the 'sticky' at the top of this very forum:

https://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

 

[Mosaic]

Thx Nigel, perhaps you can consider adding this link to the sticky...it discusses and rates all different kinds of opamps for audio purposes..... It rates the TL071 quite poorly.
https://tangentsoft.net/audio/opamps.html

and this link discusses the source follower for HI SPL AG mentioned.
**broken link removed**

**broken link removed**

 

[audioguru]

Thx Nigel, perhaps you can consider adding this link to the sticky...it discusses and rates all different kinds of opamps for audio purposes..... It rates the TL071 quite poorly.
https://tangentsoft.net/audio/opamps.html

I saw this article before. It describes opamps that are overloaded because they are driving 33 ohm headphones. Most opamps are spec'd with a 2k minimum load resistance.

The article was written by an audiophile who is actually an "audiophool" (fool) with words describing the sound of opamps that should not be used to describe sounds. All the opamps have an absolutely flat frequency response from DC to green light so they all sound the same and their distortion is so low that it certainly is not heard. An opamp was described to produce a "tubby" sound. Tubby??

 

[Mosaic]

Ok thx,
ag...I found this circuit for the OPA2134 , how could I go about making it single ended to run with the power amps 19VDC supply?
**broken link removed**

Edit: having done some more research, it seems there are good reasons for leaving the split supply as is. Perhaps the better approach is to use 2 isolated supplies , a dc blocking cap and tie the 'grounds' together to let the preamp drive the power amp.

This is an interesting link on improving the power amp:
https://www.trevormarshall.com/class-d-tutorial/

 

[audioguru]

The preamp using the OPA2134 can use a single-ended supply if the opamps are biased at half the supply voltage and use capacitor-coupling.

It is interesting that the Chinese class-D amplifier had terrible distortion and was unstable. Fixing it was easy.

 

[Stan Smale M.D. (stan)]

Hi !
To everyone interested in/ discussing electret preamp/ (etc) circuits.
Just a quick question / thought.... how about adding a simple AGC circuit to
the electret preamp ? Complicates things a little, but takes care of a lot of the
problems that are being voiced.... can be something as simple as bleeding off
a little signal from the preamp (or later), rectifying the signal, averaging it with
a simple RC filter, and using this 'running average' to drive the gate of an FET.
The FET's function is to bleed to Gnd some of the signal from an 'early' part of
the circuit / amp, thereby decreasing overall gain, and maintaining a reasonable
output level for very faint sounds, and for loud , 'overdriving' sounds (assuming
adequate gain is present to pick up the faint sounds). Anyway, my two cents worth...
thoughts ?

 

[audioguru]

A compressor/limiter circuit is used in many sinewave oscillator circuits, hearing aids and radio and TV stations.
When used for music it causes the background sounds and voices to pump up and down in level driven by the beat or bass of the music.