Cutterhead ohm rating and db input level

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Elsuprimo
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Cutterhead ohm rating and db input level

Post: # 47925Unread post Elsuprimo
Wed Sep 27, 2017 2:10 am

What DB input would be suggested with a four ohm cutterhead?
Thank you for any advice.

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markrob
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Re: Cutterhead ohm rating and db input level

Post: # 47930Unread post markrob
Wed Sep 27, 2017 8:59 am

Hi,

dB as a measurement value is meaningless if not referenced to something. What you really care about is power applied to the head. That can be expressed in dB if its referred to some operating level. For example 0 dBm is defined as .001 watts. In this case, 1 watt would be +30dBm. The drive level to your head would also depend on it power handling capability and impedance. Where in your signal chain you plan on making the measurement is also important. So, we need more information about your setup to make any recommendations.

Mark

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Elsuprimo
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Re: Cutterhead ohm rating and db input level

Post: # 47933Unread post Elsuprimo
Wed Sep 27, 2017 12:00 pm

Thanks Mark,
I have a Presto 6N with the 1C cutterhead. It was originally rated at 500 ohms but I had it modified to 4 ohms so I could reference the speaker output of various amplifiers. The original manual requested an optimal 20 DB input to the head. Now that I have modified the head to 4 ohms, I wonder what the optimal DB input should be? I will use a VU meter to measure DB input between the amp and the cutterhead.

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markrob
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Re: Cutterhead ohm rating and db input level

Post: # 47937Unread post markrob
Wed Sep 27, 2017 3:11 pm

Hi,

I don't have a 1C manual, but I do have the 1D. They state and operating level of +23 dB (referenced to 6 milliwatts) and +31 dBm (referenced to 1 milliwatt). These both represent about 1 watt drive. Note this is at 1Khz. Not stated is the resulting velocity, but we can assume its in the 5cm/s rms range. If you really want to get a handle on things, buy a test record and see how much drive it takes to make the head cut 5cm/sec at 1Khz. If you place the VU meter at the line level input to the power amp, you would apply a test signal at 1Khz and determine the level at the amp input that achieves these velocity. Calibrate the VU meter to read 0 at that point and you are good to go. Do not change the input gain adjustment of the power amp (if one is present) once you lock this in.

Mark

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Elsuprimo
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Re: Cutterhead ohm rating and db input level

Post: # 47939Unread post Elsuprimo
Wed Sep 27, 2017 3:43 pm

Thank you very much. A test record, I have many, is an excellent idea.
Where would you put the reverse RIAA filter in this setup?
Can you recommend a ball park +db level to match a 4 ohm head before I attempt to calibrate to your parameters?

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Elsuprimo
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Re: Cutterhead ohm rating and db input level

Post: # 47942Unread post Elsuprimo
Wed Sep 27, 2017 5:01 pm

Is the reduction in ohm rating and +DB rating proportional, bringing the db input to approximately zero?

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markrob
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Re: Cutterhead ohm rating and db input level

Post: # 47945Unread post markrob
Wed Sep 27, 2017 8:39 pm

Hi,

You should keep your RIAA in the path so its contribution to the overall gain path is included. 1 Khz is typically the unity gain reference point on the curve, but not always. Make sure you only calibrate at 1 Khz so the boost or cut of the RIAA is not in play. At 4 ohms, you would look to apply a 2 Vac RMS signal right at the head to produce a 1 watt drive level. I would start there and see how the cut velocity compares to your test record. If, for example, you find the test record is 2 dB hotter on playback than the cut, then you would have to increase the voltage to the head by +2db = 2.52 Vac RMS. This would become your reference 0 dB VU cal point.

Mark

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Elsuprimo
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Re: Cutterhead ohm rating and db input level

Post: # 47946Unread post Elsuprimo
Wed Sep 27, 2017 9:26 pm

Thank you so much.
I am grateful for your help.
This was the last chunk of info needed to optimize my cuts. From here I learn the ropes through trial and error.

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