The Secret Society of Lathe Trolls

cuttercollector · Joined: 11 Jun 2006 Posts: 241 Location: San Jose, CA

On that subject, a little experimentation with a speaker instead of the head is advisable. If a signal is too weak, applying excess gain in later stages causes more background noise. If too strong, the signal may clip on peaks. In the case of multiple volume control adjustnemts between input and output, my general rule has been to avoid extremes. If anything has to be turned up all the way or just barely on, you have a gain staging issue. If the overall gain is too low, as it sounds like it is in your case, I would carefully add one more device. Set the earlier stages as loud as you can without them clipping on peaks - 75% +/- and if you are using a stand alone outboard eq, be sure to use any overall makeup gain it might have to compensate for your eq curve (which should mostly be cut).
If all this is in your computer, all the same things tend to apply with the built in "volume controls", but you are at the mercy of your audio card's output level to drive the power amp. Sometimes NOT a good combination.
As I was saying, some stereo preamp to power amp interfaces in the old days operated up in the 1-5V region! Your sound card would be good to produce .5V before clipping. Also, if you are driving an old tube amp, the input impedance is pretty high, so your source needs to be able to develop the needed voltage for full rated amp output across 100Kohms or so.
Or, if your 100W power amp you cut with, is a newer solid state unit, it will not develop probably more than 75 W across your 16 ohm cutter.

JayDC · Posted: Thu Nov 08, 2007 9:49 pm Post subject:

yeah, it solid state, and thats why I'm think of going to 200w, so I'll have the extra room. it would only be putting out about 140w @ 16ohms.

Doug 6N · Joined: 21 May 2007 Posts: 57 Location: Washington

Hi Jay:

I'll weigh in with a few coments and cuttercollector you can correct any possible errors. Smile

Re: The 1C Head. 0DB is equal to 4 volts at the head into the 15 ohm cutter.

The actual impedence is not constant over the frequency range in reality. Also the 1C Head when used at 15 ohms should have a 3 ohm resistor in series with it. This is prevent the lower frequencies from overloading the amp as regards to frequency and somewhat modify the response of the cutter. The head also has a built in mechanical filter the rolls of the low end at roughly 6Db per octave from the turnover point of 500 cycles. That approximates the RIAA low end rollof. Back to that 3 ohm resistor. It adds a small amount of further reduction in the low end and then is darn close to the RIAA low end specs.

Most solid state amps do not have an output transformer to supply solid termination to the load. I'm certain you should be using a power amp with an output transformer and various taps to select the impedence in use. Cuttercollector here's an input spot for you. Smile

I've only dealt with proffessional power amps and the ones I have installed usually take 1 to 3 volts input level and generally have an input gain control to adjust the gain up or down from the reference level.

Impedence mismatch can cause huge losses. IE. 10K into a 100K load can result in considerable gain loss and modification of intended frequency range. 8 ohms into 16 ohms is about a 50 percent loss.

I welcome any comments from cuttercollector as I'm not real good at explaining all this. Smile

The biggest amp Presto sold to use with that cutter is 60 watts Many of their amps were only 10 watts like the 90B I got from you. Smile

If the magnet in head is weak. It will not give the desired gain no matter what you drive it with.

Ok! That's my two cents worth. Very Happy

Doug

cuttercollector · Joined: 11 Jun 2006 Posts: 241 Location: San Jose, CA

Jay, can you tell us how much of this (if any) is internal to your computer.
Forgive me for being old but I am always thinking of an analog program source output hooked to various boxes in an analog way. Like a r2r tape machine feeding a mixer feeding an eq feeding your cutting amp for example. If you are trying to do a majority of this in your computer and just hook the cutter amp to the main analog output of your sound card, as I said before, you may have issues. If your cutter amp is a modern solid state amp with no output transformer, you may or may not have enough voltage gain to drive the amp to it's full output. See if there is a spec. for your amp like "rated input voltage for full output" You will probably need something with a little more gain and output voltage capability than your sound card to drive the amp. Hence the mixer suggestion. As to the output impedance and output power to the cutter, Doug 6N is right. If you use an amp that does have an output transformer you will be able to select a 16 ohm tap and get full power. Pretty much all modern amps that use no transformer make less power into 8 ohms than 4 and less into 16 than 8. The rating now days may be for 4 or 8 ohms as most people don't have 16 ohm speakers anymore. So your 100 watt amp could be as little as 25 watts into 16 ohms if the 100 W was at 4 ohms. Still probably enough to drive the cutter though. Doug's other point is also correct in that any transducer whether it is a cutter or speaker is not a constant load impedance across it's entire frequency response and that impedance variation gives a built in eq of sorts. However, if you ad the 3 ohms in series he mentioned, it might mean less eq, but you are now looking at a 19 ohm load (or possibly even greater at some frequencies) which your amp will produce even less power into. A note here to all following this who are experimenting with the crystal cutters. These are VERY high impedance. A normal modern amp will not drive them very well at all. They were coupled either directly to the output of a tube or a special winding on a transformer designed to give them the high voltage low current signal needed for their high impedance. I have driven one by using the so called "70V" output of a PA amp designed to drive multiple background speakers with transformers.
Last, If Gib rebuilt your Presto head, I am pretty sure he would have re-magnetized it so that gain issue should be no problem.

JayDC · Posted: Wed Nov 14, 2007 5:52 pm Post subject:

You are correct. Everything I use up to the amp is on a computer. I use an RME Hammerfall HDSP 24bit D-A card. This is the best audio hardware you can get, as far as I'm concerned. I've used Protools TDM, and HD and just prefer RME over that stuff. It's way more stable on Windows.

For my audio processing and editing I use Steinberg Wavelab. This is buy far the simplest, but powerful editor/processor you can get. Forget sound forge, or the like.

then my 24bit 96K audio goes through a bit of processing:
1 Limiter set to -0.5
2 Desser
3 highpass @ 30hz
4 lowpass @ 11Khz
5 Head EQ
6 Inverse RIAA EQ

As you can see I keep the peaks @ -0.5, kill the ssssss, take care of the lows, get rid of the super highs, tune it to the head, then inverse the riaa. This is all handled in real time.

Unfortunately I cannot go past 0db for my source. Digital clipping is harsh.
This is all sent to the amp via 1/4" balanced TRS. The amp is a Technics, and that about all I know about it, since it was just laying about.

The laying about part is whats really driving me toward the new amp, but I kinda need a better reason then that to justify this purchase to the ol' ball and chain.

on a side note: I went to audio school at a neeto time, because I learned digital, and analog recording. I prefer to edit 24 track 2" tape with a razor, but lack the equipment to do it.

Doug 6N · Joined: 21 May 2007 Posts: 57 Location: Washington

Hi Jay:

I have been told that all proffessional sound cards are designed to work through a mixer preamp " Mackie " whatever. I go from my M Audio 192A to a preamp and not directly to an amp.

Doug

Karl Welty · Joined: 12 Nov 2007 Posts: 22

Heya,

It was said previously:

That is why I said, focus on getting a flat overall midrange response, prefrably by _cutting peaks_ with the eq, moving the sliders below zero.
Be very careful about applying compensating boost (above zero) especially at the extreme ends of the spectrum.

I would like to add a tidbit to this. I find 90 percent of the time it is *better to cut than boost* in almost any situation using EQ. The shift in level of a band of EQ will also shift the phase of the audio in that passband. This throws a portion of the audio out of phase with other frequecny areas.

But using cut the majority of the time (because on rare occasions you have to boost) this out of phase frequency area is *quieter* than the standard non effected audio... thus (in theory) less obnoxious.

Its a subtle difference... but sadly (for better or worse) once you start hearing the difference you'll wonder why you ever used boost.

I cant recall for sure, but werent many Altec and White system EQ's cut only ? I had some Altecs once which were such, flat to degrees of cut.

The bit about cutting pink noise and applying an inverse curve on playback is friggin' brilliant. I might have gotten there, but only after many blanks later and numerous cups of coffee. Smile

Karl

Doug 6N · Joined: 21 May 2007 Posts: 57 Location: Washington

Both white and altec made cut only equalizers they are really nice.

Yep cutting the peaks is the way to go then raise the overall level if you have to.

I've only done eq on motion picture theatre auditoriums which are much different but it takes a lot of power to raise the level of one frequency. I've seen lots of cases where on a 1/3 octave basis. 2K would be flat 4K would be plus to the max and 6K would be cut a lot. Overall this makes for weird sound although it can look pretty good the RTA.

There'd be times when I adjust eq by ear then look at the RTA to get an idea as what was now sounding better would look like and tweaking from that point.

The only old Western Electric Engineer I ever met that told me that "Audio Was An Inexact Science" I think he was right. He also said those sound waves do funny things. Right again. This guy had done early sound installations in the late 20's and early 30's. Too bad one cannot record the memory of people like that. And many others for that matter.

Doug

JayDC · Posted: Thu Nov 15, 2007 4:01 am Post subject:

rummaging through my surplus stash I found an ART Tube Parabolic EQ. It is what I needed, a gain knob. I put it in line, and cranked it up. Its giving me the voltage I need. I'm going to take some test cuts in the morning and see how it sounds. Should be good. Smile