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Author Topic: Gain required for Repro amp?  (Read 3528 times)

Offline cosmo_man

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Gain required for Repro amp?
« on: January 04, 2008, 12:34:43 AM »
Hi all,

Can someone report on the output level of the stock RS-1500 heads and the flux magnetics heads?

I'd like to figure out the gain required to get to the standard 2V rms output that most systems need.

This would be useful to know for modifications to phono kits like the seduction or DIY repro amps.

thanks,

Adrian



Offline ironbut

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Re: Gain required for Repro amp?
« Reply #1 on: January 04, 2008, 03:14:13 PM »
Here's a thread from waaay back there that has all the spec's. I can't believe I'm saying this but, lots of the basic spec's and info can be found using the search button.
http://www.tapeproject.com/smf/index.php/topic,31.0.html
steve koto
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Offline PJ

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Re: Gain required for Repro amp?
« Reply #2 on: January 04, 2008, 03:15:41 PM »
On the Bottlehead repro amps, the voltage gain to the unbalanced RCA jack output is about 150 at 1kHz, with another voltage gain of 4 to the balanced outputs. This seems to be enough to get the standard +4dB(u) at the balanced outs.

The IEC equalization requires gain that approaches infinite as the frequency approaches zero. We have implemented a gain of about 800 to the unbalanced output, which gives a low frequency corner slightly below 20Hz.

These numbers are approximate right now. The machines and repro amps keep getting sold, or they wander off to shows and take their time coming back  :^)  so I have not yet been able to spend much time in the lab with a complete setup and calibration tape. I think all the available machines are at CES this week ...
Paul Joppa
Bottlehead R&D

Offline cosmo_man

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Re: Gain required for Repro amp?
« Reply #3 on: January 07, 2008, 12:33:18 AM »
Thanks for the data Paul.

A voltage gain of 600 means a gain of 56 dB, so a fair amount of gain is required to get to a standard level.

Each system is different, but if you convert a low gain phono stage to a repro amp, you will need sufficient gain
in the rest of your system.

For example, the seduction gain is 36 dB. 

For my system, where my "linestage" is unity gain, my amps have gain on the low side, and my speakers are not
very efficient, the seduction wouldn't work well. 

If you have a linestage with typical gain (12-20 dB?), then the seduction would probably work fine. 

If you are also using a unity-gain "linestage," that is, a resistive passive preamp or buffered passive preamp,
 then a phono stage that has gain appropriate for a moving coil cartridge without an input transformer would be better. 
They exist in all-tube form, but usually they have a FET input for low noise.

Offline PJ

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Re: Gain required for Repro amp?
« Reply #4 on: January 07, 2008, 12:42:41 PM »
cosmo_man is right, the Seduction modified for tape head duties really does need an active preamp in most cases. Seduction, like most moving-magnet phono preamps, has 40dB gain at 1kHz with RIAA equalization (36dB without the upgrade), making it well suited to 5mV cartridge outputs. Tape equalization does not have the RIAA zero at 500Hz, so with tape equalization you get half the gain at 1kHz. High-performance tape heads, around 200mH, are more like 2mV instead of 5mV, again reducing the output. Finally, the IEC equalization does not include the 50Hz corner which is part of the NAB and RIAA equalizations. When we convert a Seduction to tape head duty, it must retain that 50Hz corner. Really, it would work better with a higher-output tape head around 500mH - as is specified for some other tube-based tape head preamps such as the Manley.

These gain limitations are the reasons for using an EF86 pentode for the first stage of the repro amp. It can be set up to have very high gain, nearly 20dB more than the 6DJ8-type triodes used in Seduction. While this topology would have enough gain for some MC phono cartridges it would not be sufficient for all of them; a true MC head amp might require even more gain.
Paul Joppa
Bottlehead R&D

Offline xcortes

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Re: Gain required for Repro amp?
« Reply #5 on: January 07, 2008, 04:02:02 PM »
I'm halfway building my Seduction amp and have already rewired the RS1500 so hopefully I'll be set in a couple of days and compare volume with my other sources. Luckily my Foreplay 3-eXcite 45 amps-edgarhorn combination has more gain than I need so if the RS1500-Tape Seduction combination has much less gain than my MC cartridge-Peerless 7497s-Seduction I will adjust the shunt resistors in the Foreplay to match volume between sources.
Xavier Cortes