Active filter project 90% complete.

[Nigel Goodwin]

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:

Just to keep you happy, I've upgraded two of the channels with TL071's (I happened to have two available). On brief listening tests they don't sound any different?, and I was particularly surprised that they don't sound noticeably quieter?, 741's are supposed to be pretty noisey opamps, and the 071 is a low-noise amp - but the difference seems very marginal (and I might only be imagining it!).

I'll see how it sounds at the gig next Friday :lol:

Incidently, each preamp is on it's own little PCB, and they link across, power and ground - with the output signals going via unscreened wire to the virtual earth mixer (another 741), which has a Presence control and a Master Volume. The preamp boards are silkscreen printed, and have the name 'Pink House' on them?, has anyone heard of that name?. The amp it self has no name at all, it just says 'PA 120' on the front.

 

[Nigel Goodwin]

Just to keep things updated a little, I wasn't really happy with how hot the heatsink got, and the mains transformer was too high a voltage for 4 ohms (at 38-0-38), the amp was designed to feed 120W into 8 ohms.

So I'm at this very moment, replacing the transformer with a 30-0-30 instead - which should be far more suitable for 4 ohm loads.

As a matter of interest, this also solves my question about the size and wattage of the transformer I'm removing - the one I'm fitting is 200W and is almost exactly the same size as the one I've taken out.

 

[audioguru]

Hi Nigel,
Are you using a transformer rated at 30-0-30VAC?
The amp will probably produce 100W RMS into 8 ohms or nearly 180W into 4 ohms with it.
With a 38-0-38VAC transformer, it would probably produce 150W RMS into 8 ohms or 250W into 4 ohms.

Do you think that painting the heatsink and transformer black will help cool them? The paint would thermally insulate them from good convection, but radiate the heat better. :?: :?:

 

[Nigel Goodwin]

Hi Nigel,
Are you using a transformer rated at 30-0-30VAC?

Yes!.

The amp will probably produce 100W RMS into 8 ohms or nearly 180W into 4 ohms with it.

Onset of clipping is about 196W into 4 ohms.

With a 38-0-38VAC transformer, it would probably produce 150W RMS into 8 ohms or 250W into 4 ohms.

The amplifier was rated as 120W into 8 ohms, but I need to run two 8 ohm speakers off of it! - hence the changes, lower voltage transformer and an extra pair of output transistors.

Do you think that painting the heatsink and transformer black will help cool them? The paint would thermally insulate them from good convection, but radiate the heat better. :?: :?:

The transformer seems quite cool, a good long run with the new transformer produced a heatsink temperature of 67 degrees centigrade, 10 degrees cooler than before - with a CD playing through it into a dummy load, clipping on occasional peaks.

The heatsink itself is only an aluminium plate, inside the box - with hardly any ventilation - the slimline design of the amplifier precludes doing much with it.

I'll see what happens tomorrow night at the gig :lol:

I'm considering buying a Behringer mixer amplifier, 400W per channel stereo with 6 mono inputs and two stereo ones, plus two seperate digital effects units. If I could have found one fairly close I would have bought one today, but now I can't get one till after the gig.

Oh, it's also switchmode, so it's small and light :lol:

 

[audioguru]

Hi Nigel,
Good luck for tomorrow night! :lol:

Do you have back-up speakers for all that power?
One of my home stereo speakers was nearly 30 years old when its flexy wire to a woofer let go from fatigue. Occasionally they would "pop" loudly when their voice coil jumped out of the magnetic field. I still haven't fixed it (I got the new flexy wire can't find any strong black glue) but replaced their tweeter wirewound level controls a few times.

At work dealers would bring in smoked woofers for "replacement under warranty". I had to tell them that the bad stink isn't covered. :lol:

 

[Nigel Goodwin]

Hi Nigel,
Good luck for tomorrow night! :lol:

Do you have back-up speakers for all that power?

NO!!!.

The only speakers I have are a pair of 1x12's with two piezo horns in each, the horns are wired in series (to give 200W handling), and the 12 inch units are Celestion ones rated at 250W RMS. They are also ported, and designed to be dual-purpose, either as simple PA (as tomorrow night), or as spakers for bass guitar (which is my daughters main instrument).

I've built these over the last few months, they are cunningly designed to fit in the small boot ('trunk' for our non-English speaking readers) of my Fiat Punto.

If I get the Behringer PA I'll probably build some more speakers, perhaps another pair like these? (because of their small size), or perhaps some ported 15's and bi-amp them with electronic crossovers, using the existing 12's for midrange/treble, and the 15's for bass.

But the problem is moving them and storing them!, space is always at a premium!.

 

[audioguru]

I have an 18" woofer still in its factory box. I can barely lift it. I'm going to sell it at the next local car sound pressure competition. It is probably worth a fortune to some rich kid, but I paid very little.

I call awful-sounding piezo tweeters, whistles. A guy fed one with 20kHz and measured 10kHz out. Intermodulation distortion and peaky response like mad!

 

[Nigel Goodwin]

I have an 18" woofer still in its factory box. I can barely lift it. I'm going to sell it at the next local car sound pressure competition. It is probably worth a fortune to some rich kid, but I paid very little.

Years ago I was given all the PA equipment from a local cinema that had been closed for years - it was been converted to an auction house, and the electrian doing the rewiring was a friend of mine.

The amplifier stood 6 feet tall, and was 19 inch rack - it had one big knob for volume (must have been five inches across!) - the output of it was only mono 10 watts!, it used really large old valves :lol:

Besides the amplifier I was given the speakers, these consisted of two 18 inch drivers made by BTH (no cabinets, they were just mounted on a board) - they didn't even have permanent magnets!, you had to provide a DC supply to energise the magnets!. There was also a single tweeter, this was a pressure unit mounted on a beautifully crafted wooden horn, it was latticed - I wouldn't like to have to try and make one!.

Anyway, I gave the amplifier to a collector, and we used the speakers. At that time I was involved in helping to run a rock club, we built two cabinets for the bass units, and hung the horn from the roof - we used a TV frameout transformer to generate the power for the magnets (crude, but it didn't cost anything, and it worked for years). Obviously with such old speakers we didn't have a clue what power rating they might be, but we fed them from a 125W Tuac module and they performed faultlessly for many years.

I was second DJ, the main DJ now actually lives in Medina, probably closer to Audioguru than me?.

I call awful-sounding piezo tweeters, whistles. A guy fed one with 20kHz and measured 10kHz out. Intermodulation distortion and peaky response like mad!

You can't beat the price of them though! - this is a budget job, and it's PA not HiFi. Have you seen the price of conventional PA tweeters?, they cost a fortune!.

As for 20KHz in and 10KHz out, are you sure it wasn't the amplifier?, at 20KHz a piezo tweeter is going to have a very low impedance, and could well upset the amplifier.

 

[Nigel Goodwin]

Just a quick update!.

The gig went well, and the amp survived :lol:

It all sounded fine, I didn't bother trying the vocal mikes through the 741 channels, I had two mikes so used the TL071 channels instead - may as well use them as I replaced them 8)

 

[audioguru]

Hooray!
You are your daughter's hero. :lol: