# Posting a two page schematic

Status
Not open for further replies.

#### audioguru

##### Well-Known Member
The voltage from a "9V' Ni-MH battery varies with load current so the voltage might bounce all the time. Also 9V is too high because a high output level will blast your head off. So a 5V low-dropout regulator is a good idea.

You have the coupling capacitors wrongly connected on the IC side of the volume controls instead of at the inputs to the volume controls. The volume controls are connected to 0V and make the inputs of the TDA2822M ICs at 0V DC as is needed.

#### audioguru

##### Well-Known Member
I am not sure I really need the 100uf on the output. Wouldn't hurt as the data suggests for stability but the circuit is not drawing very much to affect stability.
The datasheet for the LM2931 says the output capacitor must be at least 100uF to maintain stability. With a lower value then the regulator will probably oscillate. You want a stable voltage regulator so use 100uF on its output.

#### chemelec

##### Well-Known Member
The 5 Volt Regulator is Not really Needed, IF you put a 100 OHM resistor in Series with each headphone Output.
This will significally Reduce Noise in the headphone and also Prevent Excessive POWER/VOLUME from Damaging your EARS.

#### MrDEB

##### Well-Known Member
I assume this is what Audio Guru is referring to?

#### MrDEB

##### Well-Known Member
Am kinda lost about Audios post about "You have the coupling capacitors wrongly connected on the IC side of the volume controls instead of at the inputs to the volume controls. The volume controls are connected to 0V and make the inputs of the TDA2822M ICs at 0V DC as is needed. "
If I remove C2 and C5 the schematic is exactly? like the link to the Altoids amp I posted??

#### audioguru

##### Well-Known Member
I assume this is what Audio Guru is referring to?
No, it does not have an input coupling capacitor feeding the input to the volume control.

It should be like this:

#### MrDEB

THANKS again
hopefully I can order boards today. Itead = 10 boards for about $29 Dorkboards 3 boards =$32.50 ($5 sq inch) but can get quicker like 1 week plus they I believe are in Portland Oregon #### MrDEB ##### Well-Known Member Made changes on vol caps etc. Here is hopefully the final revision. The board design looks good but still contemplating using a To-92 case voltage regulator vers the Dpack but using the Dpack it has some heat sinking ability but that shouldn't be an issue anyway. Still contemplating mic mounting methods. Using 4 mics (two front and two back)U1.1 turns the back ones off and on.. I guess I am spoiled with being able to turn off the back mics (my$5K aids have this feature)
Going to order 3 boards and if you want one PM me with mailing address.
The board has a ground plane on the top as well as the bottom layers. Not real tight but not far apart. Total length of traces = 77.01 inches w/ 38 vias. 3.319 x 1.969 inches (84mm x 50mm)

#### audioguru

##### Well-Known Member
C5 is 1000uF instead of 100uF.
How will you connect front mics and rear mics? In parallel?
I connected about 10 electret mics in parallel for a boardroom conference sound system and they worked fine.

Thanks, but I don't need one of your pcb's. I never built anything with surface mount parts.

#### MrDEB

##### Well-Known Member
The mics are connected in parallel (one front, one rear mic for left channel and same for right channel. The slide switch disconects the rear mics for noisy environments.

#### audioguru

##### Well-Known Member
I think mics in parallel affect each other. With two mics in parallel then the sensitivity is less than with one mic but the difference is small (about -6dB).

Status
Not open for further replies.

Replies
5
Views
317
Replies
33
Views
2K
Replies
12
Views
1K
Replies
9
Views
1K
Replies
3
Views
698