Active filter project 90% complete.

[Juglenaut]

Well first off this prototype is for a school project, Radio shack PCB's suc, I did this as cheap as I could and it shows, I also spent little time engineering the layout before hand.

I used what I call progressive engineering assembly prototype.

Made on a single side pad per hole PCB that I got for free, all components except for the little blue resistors were free.

I think it looks fugly, you might too.

I soldered almost all traces by hand, the rest by jumper where room yields. Has a 2 stage transistor amp, 4 op amps, and other stuff. Mono active filter with up to 100 gain (adjustable) amplification.

The active filter passes all specs given by the instructor and is fully functional.

 

[audioguru]

Hi Juglenaut,
Congratulations for completing a well-working project! :lol:
Yeah, it looks kinda fugly, Veroboard would have made it smaller and less messy.
I hope you are not using it with wideband audio, its old 741 opamps are lousy above only about 8KHz.
Are you using Panasonic reddish-caps? I like the European blue rectangular ones.

 

[Russlk]

I think it is neat! I have built stuff that was a lot uglier than that. It works and that was the main objective no doubt.

 

[Juglenaut]

The filters band is from 650Hz to 4.2kHz. The caps are general purpose panasonic mylar, and one critical cap is a 1kv 0.1 tolerance ceramic, the blue resistors are multi-purpose millitary 1/2W .01 resistors from dale/vishay, which I got a ton for free.

The group I was in was skeptical about active filters, but I pushed for it to be this way because of the rejection characteristics. The wanted to use passives. Anyway the group leader (or he so self claimed himself) built a breadboarded version that worked sorta well, it had alot of noise, he also wanted to dictate the prototype, but that where I drew the line and took on the prototype engineering. They wanted to fit all the components that was on the breadboard, which had alot of double in-line resistors to make the right resistance, onto a 2 inch squared board!

I will do a follow up of the finished enclosed design next week.

 

[audioguru]

BTW, the old 741 opamp also has a lot of noise.
Why not use a low-noise, wideband and inexpensive opamp like the TL071? :lol:

 

[moutouloss]

Very good :!: :!:

 

[audioguru]

Hi Moutouloss,
How do you syncronize your stereo fans to keep them from going, WAHHHWAHHHWAHHH? :lol:

 

[Juglenaut]

I didn't use fancy op-amps because this is a school project and we are talking about Tech school level quaility components here. We were asked to use as many components on hand as we could. I had some resistors that were more exact than what the back room had in stock.

The only op-amps they had were the 741's and I didn't feel like starting another argument with the instructor and the group.

The circuit was tested for audio harmonics today and past within reason, it is realitivly clean. If the band doesn't go above 8kHz then why?? this doesn't have to be Hi-Fi

 

[Nigel Goodwin]

I didn't use fancy op-amps because this is a school project and we are talking about Tech school level quaility components here.

No one has suggested using 'fancy op-amps', only cheap, more modern ones, with greatly better performance (for no more money!) than a 741.

We were asked to use as many components on hand as we could. I had some resistors that were more exact than what the back room had in stock.

The only op-amps they had were the 741's and I didn't feel like starting another argument with the instructor and the group.

Presumably they must have been in stock for a LONG TIME!! - probably longer than you've been alive?. 741's were past it in the 1980's :lol:

The circuit was tested for audio harmonics today and past within reason, it is realitivly clean. If the band doesn't go above 8kHz then why?? this doesn't have to be Hi-Fi

I think Audioguru is being excessively critical of the 741?, fair enough it's an ancient device with a really poor spec, but it will probably just scrape enough performance for your needs?. As long as you fit sockets for the chips you can simply pop a better one in any time you like.

However, assuming your band isn't going to produce any sounds above 8KHz is a HUGE!!! mistake - there will be LOTS of frequencies well above that - you need at LEAST! 20KHz bandwidth, and probably more?. Even though you can hear higher frequencies doesn't mean you can't tell the difference when they are missing?.

BTW, I built a 12 channel mixer using nothing but 741's for a band I did the PA for - but that was back in the 1970's!.

Good luck with the band! - my 14 year old daughter is playing in the 'Derbyshire Young Band Of The Year' competition tomorrow night, so I'll be at the gig 8)

She's also got two more gig's this month (with a different band), the one on the 22nd April we have to provide all our own gear, PA and everything!. I'm still in the process of collecting gear, so it could be an interesting night!, I've not done any PA since about 1980?.

 

[ll81]

Hi there. This is my first post in this forum.

I like the workmanship for the board you have used. Have you ever tried Tripad board. It's like vero-board but instaed of strips it uses blocks of
3 by 1 holes in size.

As for the 741 I built an active crossover with these and I would have to say it works well. I was happy with this until I used the NE5532n op-amp which has a much higher band width.

Of course if use a high band width op-amp unless you carefully design your circuit layout you can come into problems with high frequency oscillations even if you use decoupling.

 

[audioguru]

I mentioned the poor quality of using lousy old 741 opamps because it is an active filter project which are usually used for audio, it is being built and tested by fairly young people who should be able to hear frequencies much higher than a lousy old 741 can produce and because the group leader built one that "had alot of noise". Besides its poor bandwidth, the lousy old 741 is known to produce alot of noise.

You can purchase four TL071 low-noise and wideband audio opamps nearly anywhere for about the cost of a cup of coffee, or half the cost of a cheap hamburger.
I think that school textbooks, teachers and stockrooms are way out of date. The last project I built with 741 opamps was in 1976, and have used TL071, 072 and 074 opamps ever since.

Here is the history of opamps (the lousy old 741 was introduced way back in 1968!) for those who are interested:

 

[Nigel Goodwin]

I mentioned the poor quality of using lousy old 741 opamps because it is an active filter project which are usually used for audio, it is being built and tested by fairly young people who should be able to hear frequencies much higher than a lousy old 741 can produce and because the group leader built one that "had alot of noise". Besides its poor bandwidth, the lousy old 741 is known to produce alot of noise.

The trick is to design accordingly, keeping your signal levels up to where the noise is insignificant - my PA mixer worked fine, fed from high-impedance (50K) microphones. The bandwidth of the 741 is dependent on it's gain, as long as you don't push it's gain too hard, it's bandwidth is fine.

I'm currently modifying a small PA mixer/amp (to use in the gig I mentioned on the 22nd April), I'm adding an extra pair of output transistors to the pair of 2N3773's already used. This is because it's only rated into 8 ohms (using a 38V-0-38V transformer), and I've got a PAIR! of 8 ohm speakers I want to use - giving a 4 ohm load. I will be adding current balancing resistors of course, and the existing mains transformer looks to be big enough (it's been replaced at some time). Anyone know how to estimate the power rating from the size of a transformer? (non-toroidal).

Anyway, the reason for mentioning this is that the preamp in this mixer/amp uses a single 741 per channel, and has high and low impedance inputs. It's used in inverting mode, with a 1MOhm feedback resistor, with a 68K for the high impedance input, and a 6.2K for the low impedance input. It's followed by passive baxendall (not sure of the spelling on that?) tone controls, feeding into another 741 as a mixer.

This stikes me as a LOT! of gain from a 741, and I'll probably replace them with something better at some stage, but the amp seems quiet enough?, and sounds really good!.

 

[audioguru]

Hi Nigel,
Figure that a Class-AB amp has a max efficiency of about 60%. It won't be driven to full output all the time (rock music does), will it? Calculate your power transformer's suitability from there.

The lousy old 741's slew-rate limits it max output above 8KHz, so at 20KHz its max output is only 40% of its supply voltage.
It also rollsoff the high frequencies depending on gain. You have a 1M feedback resistor and a 6.2K input resistor, so its gain is 161. A lousy old 741 opamp will rolloff at about 7KHz (the graph's gain scale is log).

Your young daughter and her friends might wonder where all the sizzle went in your sound system. It might sound something like an AM radio.
Without any gain for negative feedback, what about all its distortion components beating together making all kinds of noises in the audio band? Fanatics call it TIM (transient intermodulation) distortion.
For my daughter, I would use a wideband TL071 opamp and let her and her friends hear the sizzle. :lol:

 

[Nigel Goodwin]

Hi Nigel,
Figure that a Class-AB amp has a max efficiency of about 60%. It won't be driven to full output all the time (rock music does), will it? Calculate your power transformer's suitability from there.

The problem is I don't know the rating of the transformer, and yes! - my daughter plays rock music, songs at the gig will include their own songs, plus songs from AFI, Greenday, Muse, plus bands I haven't heard of :lol:

I've been running it for the last couple of hours, at fairly high volume (into dummy loads, with a speaker tapped off the bottom). I've got the chassis out, and the heatsink (just an aluminium plate) with the two original 2N3773's on, plus the two I added, is running at 77 degrees centigrade, 170 degrees fahrenheit - pretty damn HOT!. Still it's survived!, and (hopefully) it's had a LOT more hammer than it will at the gig?.

The transformer seems happy, you can rest your hand on it and it feels warm, not hot - touching the heatsink (never mind the transistors!) isn't a good idea :lol:

For my daughter, I would use a wideband TL071 opamp and let her and her friends hear the sizzle. :lol:

It's not a question of friends, it's a question of the audience at the gig, we've no idea how many might be there, we only know it's arranged by the Derbyshire Youth Service, who wanted an all girl band?.

Opamp changes are in the list of 'things to do', but for now it sounds fine, I've also got an older 300W mixer/amp, which doesn't use opamp's in the input channels (only for the mixer itself) - but the inputs don't seem as good as the 741 version, and although it has a hi/lo switch on each channel, it doesn't alter the input impedance, just the gain (I drew the circuit out the other day).

I'll be taking this amp as a backup :lol:

 

[Juglenaut]

Well then, it is not me you have to convince. I will stand up to that.

This is just for audio range, built to pick up sound produced by a harmonic chime music box. It is mono, unbalenced sound and any frequencies produced by the music box above audio is not necessary.

It is fitted with sockets, believe me I had that in mind as a requirement, also has a few swapable resistors. I also got the amp part to gain 200 on 10 mV without clipping, where as the breadbord was limited to 100. I also changed the impedances in and out to work alot better, and believe me that was even a debate. This filter and amp is only about 2 watts if that.

I did try some better op-amps after it was done, but it still picks up some radio frequency noise from close proximity switches like a transformer turning on or off. Any ideas on how I can get rid of line noise kickback??

 

[audioguru]

the heatsink (just an aluminium plate) with the two original 2N3773's on, plus the two I added, is running at 77 degrees centigrade, 170 degrees fahrenheit - pretty damn HOT!.

Hi Nigel,
I guess it needs a fan. :shock:

but for now it sounds fine

Maybe you haven't heard "sizzle" for years, and don't know what you're missing, he, he! :lol:

I'll be taking this amp as a backup

Good idea!
A few years ago, the DJ's amp at my company's Christmas party didn't work and he didn't have a backup. I cut off the RCA's from his turntable with a dinner knife, stripped the wires with my teeth and jammed them into the mic XLR jack in the banquet room. My whole company cheered me because it worked, but it sure sounded bad. :lol:

 

[Nigel Goodwin]

I guess it needs a fan. :shock:

There's not really anywhere to fit one!, I was thinking of it, but really it could be with better heatsinking than a flat aluminium sheet, bent at rightangles and rivited to the back of the chassis.

but for now it sounds fine

Maybe you haven't heard "sizzle" for years, and don't know what you're missing, he, he! :lol:

Actually, I do have very limited high frequency hearing, back in the late 60's at school we did tests with a signal generator feeding a speaker, I was the lowest in my entire year - couldn't hear above 13KHz.

I'll be taking this amp as a backup

Good idea!
A few years ago, the DJ's amp at my company's Christmas party didn't work and he didn't have a backup. I cut off the RCA's from his turntable with a dinner knife, stripped the wires with my teeth and jammed them into the mic XLR jack in the banquet room. My whole company cheered me because it worked, but it sure sounded bad. :lol:

Nice one!.

Personally I'd have been happier with the peace and quiet :lol:

 

[Juglenaut]

Thanks for the fucing positive crap fucers **** u all and this will be my last post, I will find somewhere else.

And for the one who were actually positive leave now.

 

[pittuck]

Wow. neone would think he payed for your advice...

 

[audioguru]

Hi Juglenaut,
Congratulations for completing a well-working project! :lol:

Wassamatta with him?