reviewing a preamp design

[Enzy]

Ok thanks for the info. Why do you say the speakers are lightweight, Some people use some real heavy weight speakers with good size magnets and cone the speakers over themselves, or what would you consider powerful

- - - Updated - - -

I made an update to my preamp, how does it look



- - - Updated - - -

this is based on the info I got and it seems I have filters at each end of the circuit, with the 10k resistors and the 0.47uf cap doesnt that make a low pass filter @ about 33hz?

and the end with (3.3uf and a 50k pot) is a set up like a high pass filter but still with only 1hz passing through it?

- - - Updated - - -

for higher mid frequecies up to about 1kz and high wouldnt it be better to use a 100k pot and a 2.2nf capacitor at the output? At the input I dnt see the relevance of the 0.47uf cap

 

[Audioguru]

Ok thanks for the info. Why do you say the speakers are lightweight

In the ad from the manufacturer, in the first line they say it is lightweight.

I made an update to my preamp, how does it look

Its voltage gain is about 34 times so if its volume control is set high then the output of the preamp will be clipping. Doesn't the Chinese amplifier have lots of gain and a volume control?

this is based on the info I got and it seems I have filters at each end of the circuit, with the 10k resistors and the 0.47uf cap doesnt that make a low pass filter @ about 33hz?

Bass is usually mono on both channels so then you have 5k into the 470nF= -3dB at 68Hz. It will still have plenty of output at 1kHz because it is very simple.

the end with (3.3uf and a 50k pot) is a set up like a high pass filter but still with only 1hz passing through it?

It passes all audio frequencies including very low earthquake frequencies, but it blocks the DC at the output of the opamp from damaging the pot.

for higher mid frequecies up to about 1kz and high wouldnt it be better to use a 100k pot and a 2.2nf capacitor at the output?

Then you have a highpass filter that cuts all frequencies below 727Hz. Your 18" woofer needs a lowpass filter not a highpass filter.
You do not want a woofer to produce midrange frequencies. That 18" woofer has an awful peak in its frequency response at 1.5kHz so you want to filter it out with a sharp lowpass filter.

At the input I dnt see the relevance of the 0.47uf cap

It forms a very simple lowpass filter with its -3dB at 68Hz. A simple filter like that one begins cutting at about 14Hz and still produces plenty of output at 1kHz. You want a sharp active filter that does not cut bass frequencies but has a sharp cutoff to completely cut frequencies above about 100Hz.

 

[Enzy]

I am not playing bass, you were speaking of wattage and I quickl made a google search to find soething I thought was powerful I am playing high mid range speakers and tweeters and horns with my set up.

My reply to you back then was about your statement about not needing high wattage because it will be too much and I was saying sound systems in m country use very high wattage amplifiers on sound systems to push heavy duty speakers apparently I dont know them that well out of y head I would have to look and see them and tell you.

But even if what I should you was not high wattage amplifiers and speakers if you happen to know any high wattage speakers say 2000w what if you were to play 32 2000w as bass speakers as an example for a large crowd, so you know you would want them to be driving hard what kinda powerful amplifier would you think could handle that?

- - - Updated - - -

I changed the amplifiers I am using now to 2 tda7294 amplifiers using the same schematic as the datasheet, and I am using the amp to play 12 tweeters, 4 horns and 4 12" high Mid speakers

 

[Audioguru]

A 12" speaker is a woofer and has a peak in its frequency response at about 2.5kHz. If you use a 3rd-order active filter (-18dB per octave) with its -3dB cutoff at 1kHz then the peak in its frequency response will still be heard as an odd sound. Therefore a woofer should only be a woofer for low frequencies, not also for midrange frequencies.

I do not know about very high power speakers and amplifiers.

The TDA7294 is a stereo amplifier with two channels. It produces 70W into 8 ohms per channel or 35W into four 4 ohm speakers so you do not gain power by paralleling 8 ohm speakers. But you say 2 of these stereo amplifiers will power 12 tweeters, 4 horns and 4 woofers but then the amplifiers will be overloaded unless you connect some of the speakers in series. Series-connected speakers are not damped by the extremely low output impedance of a modern amplifier so they sound boomy.

 

[Enzy]

OK how does this connection look, tweeters are 8 ohms and horns are 16ohms each.

 

[Audioguru]

You have tweeters and horns in series which causes their resonance not to be controlled properly.
You do not have a crossover network so the tweeters and horns will be destroyed by wrong frequencies.

Why do you have so many speakers? Since each TDA7294 amplifier channel produces only 70W then each tweeter gets only 70W/8= 8.75W and each horn gets only 70W/4= 17.5W.

 

[Enzy]

OK definitely that's a low amount of power, I was working off the idea that tweeters don't need much to play them, I can use 4 tda7294

- - - Updated - - -

Tweeters would be playing at 4ohms also, and horns at 4 ohms, I'm building it for someone, it's not for me,

- - - Updated - - -

Tweeters would be playing at 4ohms also, and horns at 4 ohms, I'm building it for someone, it's not for me,

 

[KlausST]

Hi,

From my experience: connecting speakers in series is no good idea.
This is true even if they are the same type of speaker, because every influence (speaker resonance, box resonance, acoustic feedback, distortion...) will immediately influence the other speaker, too.
The sound is "weak" somehow.

But connecting different speakers in series is a "no go" in my eyes.

Klaus

 

[Enzy]

So it's ok to Parrallel speakers but not connect them in series.

So then doing a series parallel connection isn't a good idea either?

 

[Audioguru]

Speakers in parallel are fine if they do not overload the amplifier. The TDA7294 amplifier is designed for an 8 ohm speaker but a 4 ohm speaker can be used for the same output power but more heating.
Series or series/parallel has speakers in series and causes resonance problems. Old guitar speakers were in series because the players and listeners loved to hear the awful sounds it produced.

Sorry, I was wrong. The TDA7294 is a single mono amplifier, not a stereo amplifier. Most power amplifiers produce almost double the power into 4 ohms than 8 ohms. This amplifier doesn't.

 

[Enzy]

I am getting distorted signals out from the preamp, Also I do not need to play frequencies from about 750hz up so I guess I would need to remove the 0.47uf cap and change the 50k pot on the output to a 100k pot.

How it is now it is distorted when I increased the 20k pot above half or even the volume nob above half.

 

[Audioguru]

With the two 10k input resistors, the 0.47uF capacitor creates a -3dB frequency at 68Hz and a gradual reduction of higher frequencies. It is such a simple filter that it begins cutting frequencies above about 14Hz. You should use a 3rd-order Butterworth lowpass filter that has a flat low frequency response up to about 200Hz then a sharp cutoff of higher frequencies. The filter does not cause distortion.

Your distortion problem might be because you have too many volume controls. The Chinese amp volume control is turned down too low and/or the volume control at the output of the preamp is also set too low. Then for enough output the input volume control is set too high so that the output of the preamp has severe clipping. Turn down the input volume control and turn up the Chinese amp and output of the preamp volume controls.
The distortion might be that the Chinese amp is garbage or it is overloaded.
If you change the 50k volume control at the output of the preamp to 100k it will make no difference to audio frequencies.

Your schematic does not show a 20k pot which I assume is R9 at the input of the preamp.

 

[Enzy]

Yes R9 is the 20k input pot, also I am not using the Chinese amp now I'm using the TDA amplifier. I removed the 0.47uf cap to remove the low pass filter and the sound was good coming from the amplifier again as I said I do not need this amp to play bass so I don't need a low pass filter.

I'll however make a bass amp later this month but I'm still searching for a circuit, that's why I was asking for help.

- - - Updated - - -

So I'm basically ok with this preamp, I however would need a amplifier design to play bass could I make a new post for it.

 

[Audioguru]

I removed the 0.47uf cap to remove the low pass filter and the sound was good coming from the amplifier again as I said I do not need this amp to play bass so I don't need a low pass filter.

But you said, "Also I do not need to play frequencies from about 750hz up" which means you want bass but do not want higher frequencies. You should have said that you do not want frequencies below 750Hz. Then you need a highpass filter.

Since you have an amplifier and speakers for it to play high frequencies and another amplifier and woofers for it to play low frequencies then you need a crossover network. Two-way and three-way speakers use inductors and capacitors inside so that the mids and tweeters produce only high frequencies and so the woofers produce only low frequencies. But your speaker types are separate and you have separate amplifiers for them so you need an active crossover network made with opamps and capacitors. Usually a 3rd-order Butterworth highpass filter is used for the mids and tweeter amplifier and a matching 3rd-order Butterworth lowpass filter is used for the woofer amplifier. If only one capacitor is used as a highpass filter then the tweeters will probably burn out from damage caused by low frequencies that are not properly filtered out. If only one capacitor is used as a lowpass filter then the mid frequency peak from woofers will sound awful. Distortion also kills tweeters since distortion is added high frequency harmonics.

 

[Enzy]

I try to build these small amps more times for people also it increases my knowledge even though I'm not switching designs often, so it's like I'm on repeat with these projects but I learn as I go since I have to mix this which is my hobby alone with my work and at times they go hand in hand.

When I mentioned bass I was only giving an example, I tend to reply in a hurry wen im replying here which I need to stop.

OK now I'm using 2 TDA 7294 chips for mid speakers I'm looking at 1khz and above frequecies and also for tweeters. Im helping someone out so I don't plan to build the cross over stage but I really wish to build those stages for my own projects, which I have 3 projects later this month, just awaiting parts.

What I noticed so far is that I'm getting saw tooth looking sinewave from the preamp not sure what's causing that, I tend to set the 20k pot at the input at about half or a little less then turn the 50k up no matter how I adjust it i still see bad waves which is giving out between, 40mv and 560mv being that it's a cheap preamp deign I don't think that's bad but I just dnt like the wave form I hear a slight distortion in the music because of it.

- - - Updated - - -

A next big problem I just encountered also is that I now connected fans as u will see in my pics, now it's giving a horrible feedback through the preamp into the amplifier how can I get rid of the noise

- - - Updated - - -



The board with the heatsink is the amplifier and the other 2 boards are the preamp boards

 

[Audioguru]

Your preamps have the parts very far apart then the wiring is antennas that pickup all kinds of interference (from the fans, from electricity and from radio stations).

A TL071, TL072 or TL074 opamp has distortion of only 0.03%. Some people can hear 0.3% distortion a little which is 10 times higher and a sinewave on a 'scope looks good up to 3% distortion which is 100 times more than the opamp produces.
Since your 'scope shows a sawtooth instead of a sinewave then the distortion must be 30%. Maybe you bought fake or defective opamps from ebay?

I designed a preamp with equalizer for a new little speaker and used TL074 quad opamps. 30,000 were made, tested and sold. Each one was tested for less than 0.05% distortion, noise less than -80dB and accurate frequency response. Only two failed, one had the IC mounted backwards and the other had a defective electrolytic capacitor. I bought the opamps from Digikey and they were made by ON Semi.

 

[Enzy]

I'm trying hard to do pcb designs also, self teaching myself, it's really hard to do single sided routing of wires and keep comments close, I could probably do it if I had alot of time to sit around and do so but I don't.

I have about 7 Tl071 all of them see giving the same wave form out I would say the components are too wide apart. Is there anything I can do to reduce hum from fan.

 

[Enzy]

I'll have to redesign the placement of the components and build some new boards, Im getting hums from the preamps because the main amplifier boards don't hum if I bypass the preamp boards.

So based on the diagram I think you mentioned that my voltage gains are too high, is that controlled by reducing value of the negative feedbacl resistor?

 

[Audioguru]

In post #21 the preamp schematic has its gain at 1+ (330k/10k)= 34 times which is low for a microphone but is high for line level. It has a 3.3uF capacitor to ground in series with the 10k resistor and if this capacitor is mounted with reversed polarity then it might be causing your severe distortion. The value of this capacitor is so high that the opamp passes earthquake frequencies. I would increase the resistor values 3 times (to 1M and 30k) and use a 330nF or 470nF film capacitor.
For a gain of 11 times then use 300k and 30k resistor, and a 330nf or 470nf film capacitor.

The input and output cables must be shielded audio cables to block mains hum pickup.

 

[Enzy]

I made a correction to the schematic, view it and verify.



I am using shielded cables also the 3.3uf caps the negative is connected to GND.