Tape Seduction eq question

[xcortes]

How is the value or R2 (the 1.5 k resistor in the NAB eq) calculated for a different high frequency roll off point? In particular I want a 70

[xcortes]

I posted this a while back when PJ was on vacation and then I turned to other projects. But I want to get back into this so I'm giving it a bump.

Thanks!

Here's a link to an interesting article on equalization. The 70

[Paul Joppa]

IT's very simple in the Seduction, where the input and output resistanceas have been chosen to remove themselves from the equalization calculation. The time constant is resistance times capacitance, so

R = TC / capacitance

So if C is 0.033uF then 50uSec is 1515 ohms, 70uSec is 2121 ohms, etc.

[xcortes]

Thanks Paul

IT's very simple in the Seduction, where the input and output resistanceas have been chosen to remove themselves from the equalization calculation.

Hmm, I can infer from that sentence that it's gonna be harder for the Eros. I'll leave that question for the near future then!

[xcortes]

The experimentation started yesterday. I carefully recorded a very good sounding tape on a Nakamichi CR-5A biased for the specific tape used. Then I rewired the output head to a new set of RCA jacks and connected them to my Tube Repro (not the required equalization yet but close enough for this stage of the experimentation).

The sound is very nice, detailed and warm. As close to tape as I can get. BUT the signal level is very low thus making the S/N ratio to high levels. I have to turn the volume and, as quiet as the Repro is, it becomes noisy. I have very quiet EF806 tubes.

I tried SUTs as I did when trying my Tascam BR-20 (where they work very nicely) but the sound became mudded. Then I studied the CR-5A schematics and saw that the load to its heads is 100k. These are high impedance heads so it's obvious SUTs are not an option.

This morning I did some research and already found that:

The output signal level from a cassette tape head is typically about 500 micro Volt or so at middle audio frequencies for a mono head and about half this level for a stereo type. The preamplifier must, therefore, provide a considerable amount of voltage gain in order to match this to a hi-fi amplifier, since these require a signal level about 1,000 times higher.

According to this the signal is 250 microVolt or 0.25 mV. So we have a head with the output level of a MC cartridge but the impedance of an MM one.

Interesting challenge!

[Doc B.]

Wow, that's really low. Looks like you need about another 10dB. Maybe a FET step up stage? Battery powered?

[xcortes]

I hear you Doc. There's a battery powered fet "headamp" for MC cartridges designed by Thorsten Loesch and with some happy builders at the Vinyl Asylum.

Another alternative is trying a tube gain stage. This guy has been succesful at building a tube MC preamp. I could try to replicate the first stage only (6c45p) and put it in front of a Seduction or an Eros. If it promises I could even try WE 437As later:

http://www.intactaudio.com/forum/viewtopic.php?p=4815&highlight=#top

[Paul Joppa]

I agree, your best hope for a quiet tube amp with 250uV input would be a high transconductance triode. Unfortunately they are usually touchy - unstable with high grid impedance, they often suffer low-frequency noise and random drift, and the tight grid-cathode spacing amplifies sensitivity to microphonics. At radio and VHF frequencies, the best tubes are quieter than the best solid-state devices, but in practice at audio frequencies it's usually the other way around.

Incidentally, the Repro and other tape preamps of ours have no high-treble equalization - it's not needed with a wideband tape head at 15ips. But at cassette speeds, the necessary head gap becomes smaller in proportion to the speed, which also reduces the available output (along with the narrower track). So there is a good chance the the head gap is more of a compromise, and some HF boost could be needed.

[xcortes]

I spent about 15 hours in the past two days building a 6c45p preamp. Very good stuff on the PS including  MQ Rs 080 and two exo 01 plate chokes per channel. The cassette sounds very good but there's a very loud hum. Louder than the noise I was getting before. So I connected the deck again directly to the Repro and started playing with grounding the deck at different screws. And at one of them, voila, noise gone. Well, maybe not gone but reduced to acceptable levels. So I increased the gain with a Quickie and the sound is very good. Noise level is very similar to the noise level of the cassette itself. It promises a lot. The sound is smooth and liquid. Kills every digital and, with tape sources, even vinyl due to the lack of clicks and pops. I'm enjoying it a lot. I may improve it someday but I can tell you one thing, cassettes are here to stay!

[ironbut]

Great stuff Xavier!
I used to listen to cassettes quite a bit. I'll see if I can dig some up and send a few to you.