Blown Fuse in Amp Problem (newbie)

[electrokinesia]

Hello Everyone,

I've been reading this forum for a couple years now as I've slowly been building up my cutting system. Thanks very much to all of you for sharing your valuable wisdom: you are keeping the art of cutting alive, and it's much appreciated.

I have a Rek-O-Kut TR43H, a Grampian cutter head and Presto 1-C. I am attempting to cut modern "dance" records which require higher wattage.

Now that I finally have the time to begin cutting, I've run into a number of problems I can solve and others I haven't been able to figure out yet. I appologize in advance for what will no doubt be hopelessly foolish newbie questions.

I have been experimenting with 2 different amps. The first is my regular stereo amp which is an ordinary Sony "Digital Audio/Video Control Center". The 2nd is my Grandpa's old PA amp from the 70s that he used for squaredance calling.

When I connect my HP 400E and/or a digital volt meter between the amp and the cutter head, it causes the amp to stop working. The Sony goes into PROTECT mode as soon as the HP400 is connected, but with the digital volt meter I can turn up the volume to a bit past 1 VAC before the amp goes into PROTECT and shuts itself down.

My Grandpa's old amp blows its fuse before reaching 3 VAC when both meters are connected. (It's my understanding that I'll have to be at at least 3 VAC or higher to get the volume I need out of the cutter head.)

Both amps should be more than capable of putting out the 30 watts or so that the cutter head requires to cut a modern dance record. It seems to me that it is not really anything wrong with the amps, but the connection of the meters that is causing the problem. But of course the meters have to be connected. Does anyone have any idea what I'm doing wrong here? Any help would be much appreciated. Thanks in advance!

Mike

[Serif]

Hi Mike, What if you just measure the volts with the meter across the outputs, but without the cutterhead in the loop, and thereby determine the maximum gain knob deflection / setting for the amp in chain, and then remove the meter, replace with a cutter head of known resistance and be confident that you're not pushing the head beyond its target power diet?

Is there a feedback loop in place with this setup?

Does the Grampian require that you tune the viscosity of the damping oil over time for consistent results?

Not sure about the modern dance performance of 30 W/channel. You'll have bass for days, but what about the treble junkies (;


- A. Damant

[electrokinesia]

Even when the cutter head is not in the loop it will still blow a fuse/shut the amp down if a meter is connected.

The Grampian head has feedback wires but I don't have them connected to anything yet. Not sure about tuning the viscosity of the damping oil.

As for 30 Watts, when I was just getting started gathering my gear a couple years ago that's what a pro cutter told me I'd need. Right now I'm just trying to get started at 1.5 VAC and move up progressively from there as was recommended some time ago in this forum. Thanks for your response!

Mike

[markrob]

Hi,

Are you sure you are connecting the DVM and HP voltmenter correctly? Is the DVM battery powered? Is the power amp running with a floating output (e.g. bridge mode)? There must be something dumb wrong with the method you are using to make the measurement. You indicated that you have the meter connected between the amp and head. It should be connected across the + and - teminals of the amp (in parallel with the head).

Some things to consider.

1. The orginal Grampian cutting amp provided in the US by Gotham was a 150 watt unit. I think the Brittish RA7 amp was 100 watts. That's about 5-7db hotter than a 30 watt amp will deliver. Also keep in mind that the Grampian is 15 ohms. A solid state amp rated 30 watts into 8 ohms will only produce 15 watts into 16 ohms unless it has an output transformer with multiple taps (one being 16 ohms). You will never run 150 watts into this head for anything but short duration peaks, but you'll need the headroom to get a hotter cut. The spec on the Grampian indicates that you need 1.5 Watts RMS at 1Khz to cut 7cm/sec ( a common reference level). 150 watts gives you 20db of headroom for peaks. Less if you cut a few db hotter on you average level.

Mark

2. The Presto head is simialar in spec, but probably not as capable as the Grampian for hotter levels. Again, check the impedance of your head as they were available in different versions.

[gold]

1. The orginal Grampian cutting amp provided in the US by Gotham was a 150 watt unit. I think the Brittish RA7 amp was 100 watts. That's about 5-7db hotter than a 30 watt amp will deliver.

The gain of the amplifier will mostly determine the output level. A 150 watt amplifier with a gain of 20dB will be louder than a 300 watt amplifier with 3dB of gain, on program material. The power has more to do with distortion and high frequency response when driving a cutterhead.

To the OP. What are you trying to measure? You can't measure power in watts with a voltmeter.

Markrob is onto something with the HP400. Most amplifier outputs are floating. You would be grounding the speaker output with the HP400. That will certainly blow a fuse! Use a battery powered meter. But I don't know what you are measuring and why you are measuring it.

[electrokinesia]

Hi Mark,

Yes, the DVM is battery powered. I'm not sure if the amp has a floating output but I suspect that it does: as Gold points out below, the HP400 is probably grounding the speaker output. But that doesn't explain why this is happening with the DVM. Could the DVM be grounding the speaker output somehow even though it is battery powered? Or could the + and - be connecting somehow INSIDE the DVM?

I believe I had the DVM connected correctly, coming off the + and - terminals of the amp in parallel with the cutter head: at least I didn't do something as stupid as connecting them in series haha. The mystery remains.

Thanks very much for the info about application of the Grampian cutter. Mine is a BBC type B1/A, and my DVM indicates the impedance is 6.1 ohms.

Mike

[electrokinesia]

Hi Gold

That would explain why the amp blows a fuse when the HP400 is connected, but not why the same things happens when the battery powered DVM is connected.

What I'm trying to measure is AC voltage of the speaker output so I can supply the correct amount of power to the cutter head. I've read various posts in this forum where cutters with similar set ups to mine have suggested to start at 1.5 VAC and move progressively upwards from there. The posts indicated that cutters were getting good results at around 3 or 4 VAC, so I'm trying to get my equipment to work in that range.

Mike

Markrob is onto something with the HP400. Most amplifier outputs are floating. You would be grounding the speaker output with the HP400. That will certainly blow a fuse! Use a battery powered meter. But I don't know what you are measuring and why you are measuring it.[/quote]

[gold]

What I'm trying to measure is AC voltage of the speaker output so I can supply the correct amount of power to the cutter head.

You are using power and voltage interchangeably. They are not. Power is measured in watts. P=I^2*V. Voltage is V. I is current.

I've read various posts in this forum where cutters with similar set ups to mine have suggested to start at 1.5 VAC and move progressively upwards from there. The posts indicated that cutters were getting good results at around 3 or 4 VAC, so I'm trying to get my equipment to work in that range.

I would suggest calibrating to the standard reference level. 5cm/sec is the mono standard. Get a test record with 1k Hz at 5cm/sec lateral (mono). Adjust your playback cartridge so the 1k hz tone reads 0 VU. If you don't have VU meters you should. Then feed 1K Hz into the cutterhead amplifier. Start bringing up the level very slowly. If you can't listen to playback as you record then check the level on your playback system. When the level on the disk you cut reads 0VU you are calibrated to the standard reference level. This is how you calibrate a disk recording system.

[Serif]

"Or could the + and - be connecting somehow INSIDE the DVM?"

Yes, it is a very high value resistor + a battery, basically. (; Even if you're seeing an open when looking at the probes while the meter is in voltage-reading mode, it can pass current. It's not really "open." It has to eat a little, but, in parallel, it's so little, the circuit doesn't care. In series, it has to eat the whole enchilada. !~!

As Markrob says, you will be touching the probes to the + _and_ - leads, respectively, creating a parallel path for the tiniest of trickles.

Also, as Markrob indicates, 7 cm/second is a common monaural (or lateral) reference velocity. However, 8 cm/second is also seen. 5, and 5.5, cm/sec are reference velocities of one channel of a stereo groove.

Feedback wires are used to sum the output of them with the input of the amp. The summed signal getting amplified would be mostly "dry," but you'd dial in a little "wet" back from the head (closing a loop) and the result would be to iron out the frequency emphasis that is extreme about the resonant frequency of the head. However, the feedback leads would be reversed, so that they sum out of phase with the input of the amp. There will be progressively more and more smoothing of the warped response of the cutter head (which has a complex impedance) as more feedback is applied, up to a limit. A cut sweep can determine how much feedback is necessary to achieve maximum flatness that is still well before oscillation.

The Ortofon amps I have recommend about 10-12 dB feedback be applied to the DSS 731 and 13-15 dB to the DSS 661, via the "cutter damping" controls. That's electrical damping. The Grampian you have may have both electrical displacement damping and mechanical (oil) high frequency damping. For what it's worth, I have seen notes from Sherwood Sax's RAS-240/300 amps that mention about 30 dB feedback for an SX Neumann head. Flat to 50 kHz. - 5 dB at 100 k. ): If too much feedback is added, however, there can be high or ultra frequency oscillations that are unwanted and potentially destructive.


Generally, the voltage measured across the feedback coil wires is an electrical "analog" of the stylus velocity. Since the meter will normally read the RMS voltage, one multiplies this reading by root2 (i.e., 1.41421356...) to derive the peak stylus velocity (which is, as you will recall, at the zero crossing of the cut sinus).


- Gib Levleur

[markrob]

The gain of the amplifier will mostly determine the output level. A 150 watt amplifier with a gain of 20dB will be louder than a 300 watt amplifier with 3dB of gain, on program material. The power has more to do with distortion and high frequency response when driving a cutterhead.

To the OP. What are you trying to measure? You can't measure power in watts with a voltmeter.

Markrob is onto something with the HP400. Most amplifier outputs are floating. You would be grounding the speaker output with the HP400. That will certainly blow a fuse! Use a battery powered meter. But I don't know what you are measuring and why you are measuring it.

Hi,

I was refering to the difference in power between the two amps, 10 log (P1/P2), not the overall voltage gain of each amp. Its assumed that you have anough input voltage to drive either amp to full power. In fact, most amps are scaled such that full power is reached when something on the order of 1-1.5 Vrms is applied to the input. As you state, the head has to be able to handle the peak power provided without distortion or its of no use. That was my point about the 1C. I suspect the Grampian will perform better in this regard.

Mark

[markrob]

Hi,

Not sure what is going on here. There should be no way that a battery powered DVM would cause too much loading on a power amp. These meters have no possible return path to the AC mains ground and are typically 10meg ohm input impedance. Are you sure you are not making the measurment on ohms or in current mode?

Also, note that the DC coil resistance you measure on the head is NOT the impedance. It is always higher (the DC measurment does not take into account the head inductance and back EMF). The Grampian is 15 ohms impedance and the DC resisitance of the drive coil is about 3.7 ohms (23 ohms for the feedback coil, if present). Check to make sure that there is no connection from the drive coil to the case of the cutter. If there is, then your head has a defect.

I assume you are not using any feedback with the Grampian. Is this correct? If you are, search for my post about the proper way to close the loop with a Grampian. Also note the the Grampian is not a true motional feedback cutterhead it is more like a transformer.

Here is something to try. Get a speaker and connect to your amp's output (disconnect the cutter head). Feed a 1Khz sine wave to the amp input and adjust to a good level. Now connect the DVM and measure the AC voltage present. Connecting the DVM should not change the sound output of the amp or make it go into protect.

Mark

[gold]

As you state, the head has to be able to handle the peak power provided without distortion or its of no use.

I meant that more power would get you less high frequency distortion up to the power limits of the head. That's why the later Neumann SAL74 has 600W amplifiers over the 150W VG66 when driving the same SX74 head.

Using a 7cm/sec reference level is probably better than using 5.5 cm/sec.

[markrob]

As you state, the head has to be able to handle the peak power provided without distortion or its of no use.

I meant that more power would get you less high frequency distortion up to the power limits of the head. That's why the later Neumann SAL74 has 600W amplifiers over the 150W VG66 when driving the same SX74 head.

Using a 7cm/sec reference level is probably better than using 5.5 cm/sec.

One thing that gets confusing about these reference level specs is that they often don't state if they are peak or RMS velocity. 7cm/sec peak velocity is the same as 5 cm/sec RMS. Pk = 1.414 X RMS. The peak velocity occurs at the zero crossing of the sine wave.

Mark

[Serif]

Using a 7cm/sec reference level is probably better than using 5.5 cm/sec.

It may be so, but it's also interesting that, as Markrob's RMS comments point out, 5.5 is louder than 7, since the peak value of 5.5 RMS would be 7.778 cm/sec (and 7 is not a standard RMS reference velocity). Similarly, if the single stereo channel peak (diagonal) velocity is 5.5 cm/sec (really, 5.66 cm/sec), the peak lateral velocity is 8 cm/sec. The root2 summing occurs in the 45/45 orthogeny of stereo cutting, as well as in RMS to peak monaural conversion.

Even in 1961, I'd not recommend settling on 5.5 cm/sec peak monaural velocity. (twss)

...7cm/sec peak velocity is the same as 5 cm/sec RMS. Pk = 1.414 X RMS. The peak velocity occurs at the zero crossing of the sine wave.

Mark

...which happens to happen in the center of the stylus's range of lateral deflection ~ perfect analog... (;

Indeed, 5 cm/sec is a standard RMS value reference velocity, however, when it is a peak value, it is the (diagonal) velocity of one channel of stereo recording at 7 cm/sec peak lateral velocity. Also if a stereo head is recording at 5 cm/sec RMS lateral, then its 2 channels are cutting at 3.54 cm/sec RMS, each.



- Ear Rationale

[gold]

I'm pretty sure all velocity reference levels are peak to peak. ABG said the math is much easier with peak to peak. I wouldn't know.

Since the only way to accurately measure level is with a Buchmann-Meyer light pattern, RMS wouldn't be practical visually.

[Serif]

Peak is way more common. However, r. m. s. is also seen, in both (disc) recording and repro contexts.

The 1961 CBS Stereophonic Frequency Test Record, STR 100, cites 5 cm/sec r. m. s. velocity at 1000 as "standard recording level." They add, "Thus, the voltage sensitivity of a pickup measured on the STR 100 record can be expressed in volts per channel for 5 cm/sec r. m. s. lateral velocity."

The voltmeter reads r. m. s. One multiplies the r. m. s. value by root2 and thereby derives the peak velocity.

The same is true for the feedback voltage from the head. It is the r. m. s. analog voltage of the stylus velocity. After this is converted to peak (multiplying the read out by 1.414213... a never-ending, non-repeating number), it can be compared to the B-M width...



- Not-so Special Agent .0007

[mossboss]

Nice thread
However guys!!!!
This newbie must by now loosing interest very very fast
All relevant but way past a new boy in the block stuff By now he is
Cheers

[electrokinesia]

Haha yeah a lot of this is over my head, but I look forward to eventually understanding it all

Your generous replies have given me much to consider and experiment with. As Mark suggested I connected speaker to the amp, ran a 1kHz sine wave through and then attached the DVM, which did not alter the sine or cause the amp to shut down, so there was no connection happening inside the DVM.

I continued the experiment by disconnecting and reconnecting everything, testing and re-thinking as I went along, and found that the problem was caused by converting the stereo audio source to mono AFTER the amp instead of before it, oops. Yes, I feel extremely stupid but I guess that's what being a newbie at something is all about! Hopefully if other newbies in the future are having the same problem, they will find this thread and it will help!

Now I have it set up so the original audio will be in mono (pre-merged left and right channels) and only coming out one channel of the amp. Everything works fine now with the meters connected.

Thanks Gib and Mark for info on the Grampian feedback connections which is something I was unsure of. I was under the impression it required a special type of amp in order to use. Sounds like I might be able to do it with this amp though.

Does anyone know where one can get a test record with 1k Hz at 7cm/sec?

Thanks again to all of you for your help!

Mike

[opcode66]

There are a number of places that sell this record. It is not a NAB or CBS record but it works for me and many others. I think Flo was going to make a test tone record as well. Wonder if that every came to fruition???

Anyway, you can google Ultimate Analog Test LP and find a bunch of online stores selling it. Here is one:

http://store.acousticsounds.com/index.cfm?get=detail&title_id=35532

[markrob]

Now I have it set up so the original audio will be in mono (pre-merged left and right channels) and only coming out one channel of the amp. Everything works fine now with the meters connected.
Mike

Glad to hear you were able to sort things out.

If you are running the 15 ohm Grampian and want more output power, you can take advantage of the unused second channel and run your stereo amp bridged for mono operation. If the amp is currently rated 30 watts into 8 ohms, you will only be able to output 15 watts at 15 ohms. However, if you run the stereo amp bridged, you will be able to drive the head with 60 watts of power. To do this you need to feed the mono signal to one input and an inverted version to the other channel. You then connect the head between the L&R channel positive terminals and your are good to go (the outpt is now floating with respect to ground). If you are driving the amp from a computer source, its easy to invert the mono signal and copy the audio to the other channel. If not, then you would need to build something external to accomplish this (this is probably beyond you means right now). This trick will only work if the amp is not using a floating output.

Mark

Mark