I need a bigger amp, but how big?

[kugelblitz]

This weekend i made my first feeble steps in the world of record cutting.

I had read quite a bit on this forum, which had tempered my expectancy quite a bit and indeed the first cut has me sonically getting about to par with those first edision cylinder recordings.

Loads of surface noise, and beyond the shriek of of the cutting needle the faint sound of the song i was cutting.

But hey: i started.

I’m using a french made 78t dual/carbazone machine with pierre cardin head that i bought restored. The guy i bought it from apparently modded the pitch control so severely that it turns at 33⅓.
He told me he did this by changing the size of the piece of felt that controlled the pitch on the original machine.
Quite clever, but the motor isn’t exactly silent, so i might want to change that later on for a belt system.

First step however is getting decent levels.
The previous owner told me to use 200 watts amp to drive the head, but a 200 watt amp only starts to be audible on the cutterhead-side when i open everything up completely.
Maybe he meant 2x200 watts

Reading on the forum i also see that people generally use bigger amps. So i figure i need to buy a bigger amp but how much bigger?

Head reads about 20Ω accros the poles

Head is a Pierre Cardin M1

All tips are welcome!

pic of the machine:

dual.JPG

[dubcutter89]

Hi!

Hard to say exact number of how many watt an amp need to cut a decent record, but for most cases somthing of approx. 100watt should be enough!
With regards to your system I see a couple of things:
- Is the cutting head in good working condition? That means the magnet is still strong, the coil is good and the assembly of parts is done right.
- Maybe your head is ment to be used with a different amp (output)? The 20 Ohm reading (I assume DC reading from a multimeter) indicates quite high impedance. So maybe your head should be driven of a high voltage output? Maybe you should use a transformer to fit your head impedance to the common low impedance used for modern amplications and power amps.
- Add protection fuse inline with the head. A fuse is cheaper to replace than the cutterhead...

Hard for me to imagine that this little moving iron mono head really needs killerwatt power to cut at decent volume..
Maybe check how the cutter should be driven before investing in more power.
Lukas

[kugelblitz]

- Maybe your head is ment to be used with a different amp (output)? The 20 Ohm reading (I assume DC reading from a multimeter) indicates quite high impedance. So maybe your head should be driven of a high voltage output? Maybe you should use a transformer to fit your head impedance to the common low impedance used for modern amplications and power amps.

you might be on to something!
looking for "pierre clement" (in my OP it says "pierre cardin", but that's wrong) on this forum finds only a few posts but one states:

Sometime ago I bought a Poltz Freres Lathe which had a Pierre Clement Cutterhead with it. This Pierre Clement Cutterhead has an "Unusual" DC Resistance of 23 Ohms, making it neither a "Standard" 8 Ohms ish! Cutterhead, or of the 500++ Ohms High Impedance Variety. I subsequently Hand-Wound a Transformer to Drive the Pierre Clement Cutterhead-

I made the Primary Winding 8 Ohms ish, and the Secondary Winding 23 Ohms ish figuring the "Turns Ratio" would take care of itself and end up about 3:1 and Just about Right to Drive the Pierre Clement Cutterhead from a "Standard" Amplifier. -

I have no experience winding coils, but i guess that's about to change

as for the state of the head, it is hard to judge for me for the moment. I guess we will find out once i drive the head the right way!

and i also have a inline fuse installed !

thanks for the help!

[markrob]

Hi,

You can avoid rolling your own transformer if you get an off the shelf 70.7 Volt transformer and use it backwards to match your head impedance. The 10 Watt secondary tap will match the 8 ohm primary tap at 500 ohms.

For other head impedances, use the formula: Tap Watts = 70.7 Squared / Head Impedance (5000 / Head impedance). Connect to the closest tap available.

Connect your amp to the 8 ohm side of the transformer and the head to the 10 Watt tap and its common. Here is a link to typical unit that could work.

https://www.amazon.com/Commercial-Transformer-8Ohms-OSD-Audio/dp/B007TJNP5M?th=1

If you get a beefier transformer with more iron rated for higher max power, you can get better low frequency response and avoid saturation on the high power peaks. But you will always use the 10 Watt tap to drive the head.

Mark

[kugelblitz]

For other head impedances, use the formula: Tap Watts = 70.7 Squared / Head Impedance (5000 / Head impedance). Connect to the closest tap available.

Thanks for this info!
that transformer is unfortunately unavailable in Europe where I'm at, and 70.7v systems seem to be uncommon here, but i can find similar transformers for 100v systems.

Then i have to do the math again, but your formula is completely over my head, if i understand correctly the sum is:
70,7²÷20 so:
4998,49÷20=249,9245

and for 100v it would be

10000/20=500 - right?

how do these numbers 249,9245 or 500 relate to the wattages on the transformer?

Thanks!
teun

[markrob]

Hi,

You have it right. Use the 20 Watt tap on a 100 Volt transformer to match 8 ohms to 500. The formula I gave allows you to determine the wattage tap for a given head impedance. In your formula, you used wattage in the denominator and obtained impedance. Just a bit of algebra there.

Mark

[kugelblitz]

Okay, one last thing about your explanation puzzles me: I know you can't measure impedance correctly with a multi meter like i did on my head, but i thought it did give a ballpark figure.

So if the head reads circa 23 ohms dc resistance, why should i be stepping up the 8 ohms to 500 ohms?

In the example shouldn't i be trying to connect to the 100²/23= 434-watt tab of a transformer?

As you may have noticed, i don't know anything about this stuff, but the other user that rolled his own transformer for this particular head says he used

Primary Winding 8 Ohms ish, and the Secondary Winding 23 Ohms

the secondary winding in your example 500 ohms, right? or wrong?

Thanks for taking the time to explain it to me!

[kugelblitz]

I just noticed you (markrob) also reacted on the original (6yr old) thread i am referencing to above

There you say

The turns ratio should be around 1.7:1 for an impedance ratio of 3:1 (they are related via the square root). Even though you measured 23 ohms DC resistance, the impedance at 1 Khz is likely closer to 100 ohms or more So your ratio probably did come out closer to 3:1 after all. At the end of the day the goal is to force current into the coil. That determines the force . You have to fight the inductance, DC resistance, and back EMF to make that happen. If the resistance is too large, you will burn too much heat as you froce more current thorugh the head.

So if the head impedance is +/- 100 ohms, 100v²÷100Ω=100watt tab?

I found a transformer for 100v systems that has 100W / 75W / 50W / 25W / 12,5W tabs at an acceptable price - would that fit my purpose?

I'm sorry to question this so much, but i don't really understand where these 500 ohms as reference value come from - is it some kind of standard?
Or am i completely missing the point ?
- if i am feel free to tell me that i'm in over my head and should just take advice from people who actually understand this stuff

I have to say that reading up on transformer math made me a bit less exited about rolling my own...

[markrob]

Hi,

I think you have it down. I would use the 25 Watt tap. That would match 8 ohms to 400 ohms. That should be close enough. Probably better to use that tap instead of the 12.5 Watt tap that would match to 800 ohms. If your head is 500 ohms, it would present a bit lighter load to your amp and prevent it from being overloaded. Its not that critical. You just want to be in the ballpark so that you get the best power transfer to the head.

I used 500 ohms because that was a standard for some cutting heads back in the day. To be sure, you would need to do an impedance measurement. That is typically done at 1Khz.

You could start with the 100 Watt tap (100 ohm) and see how much loudness you get and move down to lower wattage taps as needed. The step up ratio = square root of the ratio of the impedances. So for the 25 watt tap, you have sqrt(500/8) = 7.9:1 100 Watt tap is sqrt(100/8) = 3.5:1

I agree. I would go with an off the shelf transformer over winding my own unless there was no choice.

Mark