Pro Amp for home use.

[indianajo]

Well, it has been a day.
0.7 * Vpp = Vrms for sine waves. Other shape waves you have to do an integral, not very likely unless you have a machine programmed for that.
log(10)Vrms=db so that is your soft decibel. The max voltage the amp will put out, log(10) of that is your loud decibel. Signal to noise is loud decibel minus soft decibel.
The reason I didn't answer before, I'm not sure of all this. there is also a log(20) definition or something, I can't ever keep them straight. 20 is power, 10 is voltage, I don't know if signal/noise is power or voltage. power=v^2/r where r is the speaker impedance. Plus I never remember if Vrms is 0.7 Vpp or half that.
The trouble with a PA amp, it may have way more loud voltage than you are going to use, and not be quiet enough to use at 1.5 Vpp (my usual listening level) without hearing hiss during the pauses in the (classical) music. PA amp market is usually bar bands playing full throttle to the beach dancers 20 hours a day. Classical music I listen to, the hymn part of 1812 overture can be 1.5 Vpp and the cannon shot can be 40 v. I like it like that, one reason I use 600 w speakers in my living room at 1.5 Vpp. (also they are the best sounding speakers for sale in my fly-over town, and were cheap when all those bars closed after the smoking ban went in effect).
So really a comparison of the idle voltage (input shorted) with the volume set to your usual setting, versus the idle voltage of your old amp with the volume set to your usual setting, is really what one is interested in knowing for home use. IMHO, Idle voltage is not usually in the spec sheet.
What is in the spec sheet is maximum gain. This is gain with the knob turned all the way up.
As a datapoint, my CS800s is very quiet with the record player turned off. It is about the same as my ST120 one transistor pair amp. In the music room the volume knob is about 9AM. The CS800s should be a premium product, it costs about $1000 these days. I bought mine broken and fixed it. Both hiss softer than the noise of the gas pilot light on the stove in my music room. My PV-1.3K PA amp hisses a little louder, idle. The amp was a "popularly priced" model.

 

[AndrewT]

A normal DMM set to Vac is pretty terrible as a Noise measuring device compared to "real measurements". But for us amateurs it works reasonably well.

Just measure the Hum + Noise at the output with the input sockets shorted with dummy shorting plugs.

Expect readings of 0.0mVac to 0.9mVac at the output.
Any more than that probably indicates faulty wiring, or faulty amplifier.

Maximum output voltage is easily obtained from the specification.

Max output voltage into rated load equals Sqrt(Maximum Power * Rated load impedance)
eg.
max Vac = sqrt(100W * 8ohms) = 28.28Vac for a 100W into 8ohms amplifier.

S/N ratio
If you measure 0.3mVac of H+N (very approximate since we are using an unknown voltmeter bandwidth)
S/N ratio = 28.28Vac/0.3mVac = 94267:1
and expressed in decibels, take the log and multiply by 20 to give 99.5dB

Our 0.3mVac is 99.5dB below the 100W maximum output.

A reading of 0.0mVac does not mean zero volts. It actually means the average of the readings is less than ½LSB (least significant bit)
i.e. 0.0mVac actually means < 0.05mVac on a 20000count 199.9mVac voltage scale.

Thus a 0.0mVac reading for that 100W amplifier means an approximate S/N of 115dB (20log[28284.27/0.05])

 

[Peter S]

Thanks again Indianajo and Andrew.
As usual, I have to learn everything the hard way; Regarding the fans; I went ahead and wired them in series and fed them from the right channel fan control circuit. This WILL NOT work. The manual claims that fans are powered up to full, on turn on, then ramped down to a speed determined by heat sink temperatures. My amp just puts out about 13 Volts to each fan and seems to stay there. I have not had the amp get hot yet. With the two 24 Volt fans in series across 13 Volts, they are stalled for 30 seconds and then start. You told me so! I will change the fans as suggested, if I keep this amp.

After wrestling with a ground loop problem, I'm more or less happy with noise level of this amp. (I still have my non professional opinion that a scope would be great way to measure the P to P noise voltage. I am puzzled that this amp, claiming 115 db S/N, is noisier than my 40 year old Phase Linears. The Crest is 1000 wpc/ 8 ohms, the Phase is 350, that's 5 or 8 db difference at the very most is it not?. The Phase claims only 100 db S/N. In any case, I can live with it, the noise is only audible when you get close to the woofer cones.
The saga continues though. You may have concluded by now that you are dealing with a bit of a nut here, if for no other reason than building a 3500 Watt tri-amped home sound system. Now that speaker noise and fan noise is under control, I am afraid to say; the amp itself hums! The huge torroid transformer hums (more of a buzz, I'm sure you're familiar with it) I have done some investigating, there is some iron sheet wrapped around the trans, that's not rattling. I did an amature 'stethascope' (screw driver to the ear) and found it is the windings, themselves.

I am now going to loose all credibility with the experts here but here goes;
Each amp has it's own 15 amp breaker and dedicated line to the house breaker panel. All circuits are on the same side of the neutral in the panel, so all the power is 'in phase'.
This is where it gets weird; wife turns on the micro wave, 40 feet away and on different breaker, the amp's trans got noisier. I was going put shims between that iron shielding but discovered that it was not the culprit.
Has anyone got any advice, I really do not want to have the amp in the rack,
Thanks again, Peter

 

[jkuetemann]

DC on the mains will make a transformer complain and toroids are more susceptible than other types. Can happen with having various devices using switching type supplies on the household circuits. There are some articles out there on this, check ESP's site:

Mains DC and Transformers

 

[Bare]

Dunno about this.. Read the post out of Curiosity..
. I've commissioned a fair Number of 250+ k$ Home AV systems (architect) in recent years.
NONE featured 1000 w amplifications.
Simply not required in a competently designed /considered Home Theatre system... even ones with 24 powered 'Seats'.
What My setups did have (besides less than 1/2 the "claimed" watts 🙄 were dedicated Equipment rooms (large closets) with One or sometimes 2 Silent equipment fans...~3000$ ..Each.. plus similar for install costs (external intakes and exhausts).
No! not childs play.. Wanna fool around with toys for the wealthy? The cost of admission is Steep.
Seriously suggest revisiting your setup... there IS Something fundamentally/basically Wrong.
You needs focus on it.. rather than some silly Fan Noise issue

Just My 2 cents worth of experiences 😀

 

[indianajo]

I am puzzled that this amp, claiming 115 db S/N, is noisier than my 40 year old Phase Linears. The Crest is 1000 wpc/ 8 ohms, the Phase is 350, that's 5 or 8 db difference at the very most is it not?. The Phase claims only 100 db S/N. In any case, I can live with it, the noise is only audible when you get close to the woofer cones.
The saga continues though. You may have concluded by now that you are dealing with a bit of a nut here, if for no other reason than building a 3500 Watt tri-amped home sound system. Now that speaker noise and fan noise is under control, I am afraid to say; the amp itself hums! The huge torroid transformer hums (more of a buzz, I'm sure you're familiar with it) I have done some investigating, there is some iron sheet wrapped around the trans, that's not rattling. I did an amature 'stethascope' (screw driver to the ear) and found it is the windings, themselves.

I am now going to loose all credibility with the experts here but here goes;
Each amp has it's own 15 amp breaker and dedicated line to the house breaker panel. All circuits are on the same side of the neutral in the panel, so all the power is 'in phase'.
This is where it gets weird; wife turns on the micro wave, 40 feet away and on different breaker, the amp's trans got noisier. I was going put shims between that iron shielding but discovered that it was not the culprit.

As I said, signal to noise ratio does not predict what the shorted input voltage of an amp will be .
The transformer hum of the transformer is a killer on this amp, I doubt there is room to put in an e-frame triad transformer. Also the fan running with the amp cold could be more annoying. I'd say sell off this amp and buy something else.
I'd say look around for an old Peavey CS800, particularly CS800x or CS800s. I have an CS800s, it is quite silent at 1.5 Vpp, the fan doesn't run at all and the switcher supply is also quiet. As I said the hiss at normal volume, much less with input shorted, is inaudible. My speakers are 101 db @ 1W 1 m, that is extremely loud. At 16 years of age the CS800s power supply capacitors are tripping the breaker at first startup, but that is a $30 problem for a guy handy with a soldering iron.
The Peavey PV series (4, 8C, 1.3K) have a little more hiss idle, but the mains transformer is dead silent, with a copper wrapper around it. Probably buying one of those is the cheapest way to get a first class E frame transformer. I won't brag on PV series otherwise, the PV-1.3k tends to melt the PWB protection circuit land when hooked to a shorted speaker wire, or the output transistor shorts.
As far as the excess watts, I don't need 800, but I like 50 v peaks on 1.5 Vpp normal music, for a 72 db dynamic range (Cd standard) . Most Cd's don't have this much dynamic range, but some of the Telearc and other art house CD's do. I sure remember bigger volume peaks and valleys from high school band, than you get from any recorded media, and I miss the drama of all that.

 

[Peter S]

Thanks again everyone; This horse died some time ago but, the beatings will continue...

The toroid transformer seems easy to remove. (All wires on removable connectors). Could this transformer be submerged in shellac or varnish or whatever they do with motor windings? I assume this is done to 'lock everything together'. This may require some further research but noise seems to come from the windings, themselves.

Sorry if I am exhausting your patience, but I am still hoping that this Crest is a 'diamond in the rough'.
Not to get snippy, but I somehow assume that someone spending $250k on a home system, probably has the brains to not want to listen to something at 120 db +. Neither is the case here. Never the less, I am always prepared to look at my system, from the ground up, and follow all professional installation techniques. I have, in the past, installed THX theatre sound systems for Cineplex and even larger systems for IMAX theatres.

As the dust settles; Has anyone heard of dipping a transformer, or is this wishful thinking?

 

[jkuetemann]

If there's DC on your mains then core saturation of the transformer is the real issue. No amount of varnish / resin will solve the problem even if it masks the symptom. You mentioned the microwave oven causing the problem to become more pronounced, this lends credence to the notion of DC on the mains. The issue is most pronounced with toroids and high VA ratings. Try the amplifier on the other phase of your household supply to see if that is better or worse. Look at DC blocking options.

As for dipping, it can be done. It is best if you can evacuate the dipping chamber to ensure full penetration and then bake at a low temperature to set the medium. You wife will love it 😉. At best is will mask the symptom of the disease, at worst you have taken on a messy smelly task without any real benefit.

 

[Peter S]

Hi Jason, You're almost a neighbour, I'm in the Barrie Area. I have a bad feeling that you are right about the dipping idea. I followed your link and found it very informative. I still am not comfortable running the AC supply through a couple of back to back electrolytics. The author discussed values for an amp drawing 5 amps. The Crest has a special plug, 10 gauge line cord, and calls for a 30 amp dedicated circuit.
I have done some research on this dipping; it quite often calls for baking, as you say.
I think the baking would be impossible with the lower temperature insulation of the transformer leads and connectors. The center core seems to consist of a poured in black (epoxy) but the windings are just covered in what looks like electrical tape.
I was considering perforating the tape, somewhat, and submerging the whole thing in a container of varathane. I would cover the can (for a few days) so that the paint would (hopefully) displace the air before drying. I would leave the connectors above the varathane level.
Another more ridiculous, but probably more effective solution I might seriously consider, would be to mount the amp in my rack, but remove the transformer and install it in the basement. At least the power switch, panel indicators, etc would be in the rack, and signal cables etc would not have to be extended. I really don't like the idea though.
I will check the amp on other circuits, but I live in the country, and the house has it's own transformer on the pole, would it be safe to assume that the dc offset is generated after the hydro transformer ( in other words, in my house?).

It just occurred to me that my Yamaha P 2200 has a toroid transformer and it is dead quiet. I will follow up on your suggestions so far, and thanks for your advice

 

[CBS240]

Microwave mag-tron power supply and also flyback power supply for a picture tube can take more current from the positive half of the mains signal than the negative half due to half wave rectifiers and such. This can drop one polarity of the AC voltage slightly more than the other polarity leaving a small DC component on the mains signal. Also there are harmonic resonances on the mains line that cause intermittent buzzing. Toroid transformers have no gap in the core and this leads to very high flux density. Many manufacturers try to get the most out of the core and tend to operate to the extremities of the hysteresis curve. One solution is to lower the flux density a bit by adding primary turns. This also requires rewinding the secondary as the V/turn will be different. Another option might be to buck the primary with another transformer. This also requires rewinding of the secondary and my be impractical. All of this will slightly de-rate the transformer output. Of course this is impractical if the transformer is all glued together. In this case, a DC mains filter might be the best course of action here. There are plenty of examples in the forums here, just search.😉


BTW those electros are part of the mains DC filter....

 

[Peter S]

At the risk of redundancy, I would like to start another thread; "Toroidal Transformer Noise". CBS240 and everyone, please follow me there, as this thread initially addressed the fan noise issue.