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Proposed Amp Design

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Quercus

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Hello,

I've made several recent posts regarding an amp I'm building based on the TDA7294. This will be for a pair of speakers connected to two computers, plus an aux input. It has a built-in mixer to select between the three inputs, plus a low-level subwoofer output and a headphone jack. The ring switch on the headphone jack activates the mute function of the main amplifier.

The completed proposed circuit is attached. It has been mostly prototpyted on a breadboard, and I am getting ready to solder it together 'for real'. But first, I'd like to post it here for any comments.

The power on-off sequence (thank you to crutschow) turns off the standby and mute after the amp is powered on, and turns them back on again before the amp powers down. The 10,000 uF and 1,000 uF caps in the power circuit keep the amp running long enough for the power off sequence to work.

The input mixer is based on this from CircuitLib http://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer
I've replaced the trimmer pots with conventional resistors.

For the headphone output, I've tweaked the cmoy headphone amp (thank you to audioguru, Nigel Goodwin and others for your comments) to make it inverting. I haven't prototyped this part yet, so don't know if it works as expected.

The subwoofer output uses a variable low-pass and volume control, taken from this post http://www.sensibleaudio.dk/mini-sub/mini-sub.htm
My subwoofer amp actually has both, but the amp is stuck in pypass mode and is tucked way under my desk where it's inconvenient to adjust the volume - since I'm building this just for me, easier to add these functions here so I can tweak them from my desk rather than crawl under to turn down the subwoofer whenever my wife says it's shaking things too much.

I appreciate the help I've received so far, and I'm sure this amp could use some more tweaks. One thing in particular I'm unsure about is my selection of Op Amps. I've used whatever was suggested from the circuit I'm copying, so I have three separate Op Amps for three different circuit functions. I confess I understand very little about the differences between the multitude of Op Amps out there. I've read a few postings and webpages on this, but my eyes have quickly glazed over trying.

Thank you all and I appreciate your helpful comments.
 

Attachments

I don't read fully, but my first question is: What kind of Wall wart do you use? SMPS? No noise filter?
I still don't have comment about power amp and pre, mixer, filter yet.
 

Quercus

Member
The wall-wart is a basic 12V, 1 amp, nothing-special, got it off of Amazon product. I've not modified it other than fitting it into the circuit.

When I solder it together, I plan to separate the ground into three parts - one for the audio signal, one for power circuit, and one for chassis & power (e.g. electric panel) ground. They will be connected together at the audio input.
 

audioguru

Well-Known Member
Most Helpful Member
What amount of power will drive the little 6.5" woofer into the limits of its excursion? 200W? (+12dB more than 25W). Do its limits sound graceful or sound like the speaker is destroying itself?
Since you are boosting low frequencies 12dB which needs 8 times the power then mid frequencies will be reduced to only 3.125W when the amplifier is clipping producing 25W at the boosted frequency.
 

Quercus

Member
Not sure what you're getting at here, audioguru... I'm driving a good set of monitors, with 100W per channel available and 50W (approximate) demand, I'll be right in the middle range of the amp's capability which is right where I want it to be. Where do you see that I'm boosting low frequencies? The only audio frequency dependent circuit I've (intentionally) included is the low-level subwoofer output, which as noted in the OP will go into another amp to power the actual subwoofer. If you see an issue with the schematic I've proposed, I'd appreciate your clarifying and specific suggestions. Thanks.
 

audioguru

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Most Helpful Member
Sorry, I was reading about the Mini-Amp circuit where there is a small "subwoofer" speaker, a low power amp and some bass boost at the crossover frequency. Why did you post that project?
Your schematic has a grid on the background that makes confusing wiring and blurry parts values unless I zoom in on parts of it.

Problems I see on your schematic:
1) Your main speakers do not have a crossover so they will add their output to the output of the subwoofer which boosts the bass near the subwoofer crossover frequency and might cause the main speakers to have over-excursion.
2) When the power is on then the outputs of the comparators will be low then you will have muting all the time because the PNP transistor cannot produce a positive output that is needed to turn off the muting.
3) Why do you have so many opamps and why are they all inverting with a fairly low input resistance? U5, U6 and U7, U9, U10 and U11 are not needed. Simply feed the input signals to the volume controls.
4) Mixing pots R33, R38, R43, R48, R53 and R58 are 22k but are loaded with 22k. A pot that is loaded with 22k should be about 4.7k.
5) Why do you have a variable cutoff frequency for the subwoofer and so many capacitors. Set the frequency (on paper) and never change it again.
 

Quercus

Member
Thank you audioguru.

Why did you post that project?
I was giving credit to and showing where I obtained the various parts of this project. The subwoofer low pass filter is extracted from that mini subwoofer project. Sorry if this confused things.

1) Your main speakers do not have a crossover so they will add their output to the output of the subwoofer which boosts the bass near the subwoofer crossover frequency and might cause the main speakers to have over-excursion.
2) When the power is on then the outputs of the comparators will be low then you will have muting all the time because the PNP transistor cannot produce a positive output that is needed to turn off the muting.
3) Why do you have so many opamps and why are they all inverting with a fairly low input resistance? U5, U6 and U7, U9, U10 and U11 are not needed. Simply feed the input signals to the volume controls.
4) Mixing pots R33, R38, R43, R48, R53 and R58 are 22k but are loaded with 22k. A pot that is loaded with 22k should be about 4.7k.
5) Why do you have a variable cutoff frequency for the subwoofer and so many capacitors. Set the frequency (on paper) and never change it again.
1) Acknowledged. I'll consider this and perhaps build in a high-pass for the main speakers. Current plan has been to let the main speakers run their whole range and adjust the subwoofer output (volume and crossover point) to match the needs of the speakers, room and placement as-needed.
2) Thanks, will review this again and correct as needed
3) Just following the example from CircuitLib, and someone who presumably knows more about these things than I do. Purpose of the op amps as I understand it is to ensure the audio sources see a high impedance. This quote is taken directly from that CircuitLib posting:
"A single input module [the U5,6,7,9,10 & 11 from above] is a preamplifier and a matching circuit at the same time. As a preamplifier, it must be able to provide some amplification. As a matching circuit, it must have a high enough input impedance to ensure that any signal source is not overloaded. An input impedance of about 47K is considered enough high for audio applications."
4) Okay - thank you.
5) Number of caps is summing to a non-standard cap value. Reason for having a variable cutoff frequency is so that I can adjust it as needed to suit room and placement.

Thanks.
 

spec

Well-Known Member
Most Helpful Member
One thing in particular I'm unsure about is my selection of Op Amps. I've used whatever was suggested from the circuit I'm copying, so I have three separate Op Amps for three different circuit functions. I confess I understand very little about the differences between the multitude of Op Amps out there. I've read a few postings and webpages on this, but my eyes have quickly glazed over trying.
Hy Quercus,

I know what you mean about the wide selection of audio opamps and the various reviews. I have just completed a similar exercise for another couple of projects. As a result I would recommend the OPA2134. It comprises two opamps in an 8 pin pack with industry standard pin-outs. It also has a FET input with practically zero input current and low input offset voltage. In addition, it is designed for audiophile applications, with low distortion and good dynamics. These characteristics make the OPA2314 compatible with all relevant audio circuits. Unlike some audiophile opamps, it is freely available and well priced. Make sure you buy from a reliable source though, and make sure it is manufactured by one of the main manufactures, Texas Instruments for example. http://www.ti.com.cn/cn/lit/ds/symlink/opa2134.pdf

spec

PS: As far as I can tell, the current favorite audiophile opamp is the OPA627/OPA637 http://www.ti.com/lit/ds/symlink/opa627.pdf But, at £10UK for a single opamp, they are a bit pricey. :wideyed: They are also twitchy to use. The old favorites: LME49710 and LME49990 are long gone.
 
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spec

Well-Known Member
Most Helpful Member
Double post. Would Mods please delete

Thanks

spec
 
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