reverse RIAA and IEC adapter to use phono stage as head amp

[Kenkirk]

Hi,
 First post, so if this is stupid, go easy! :-)

I had a tech once make me a small network with rca jacks on it that plugged directly into my Cat's  ( preamp ) phono section. This little network was a reverse RIAA circuit and a load resistor to pad down the input so I could hook up my phono stage to a break in cd playing on repeat in my cd player. So When I replace phono tubes, I hook up this network and run a break in cd over night. Much easier than playing vinyl for hours.

Anyway, it got me thinking that maybe a little network could be made that reverses the RIAA and adds the IEC eq and maybe pads down the input some to match my phono stage on my Cat Ultimate preamp. Just wondering if anyone tried this. I will probably build a Seduction to match up with my Otari MX 5050 B III, but this might be interesting....

Thanks,

Ken

[ironbut]

Hey, a fellow Cat Ultimate owner. Welcome to the forum Ken. As you know, the phono stage on our preamps have tons of gain. I used to use a modded Grado "The Reference" moving iron cartridge (1.5mV) with no problem and have heard of guys running their .8mV moving coils straight in. I think that rather than adding an inverse RIAA and an IEC eq, it would be better to just add a switch/s and add the IEC eq and maybe even a NAB eq as well.
Are you building your Seduction yourself? If so, at least with it's passive RIAA circuit, changing it over to NAB/IEC is really simple. From what Doc has mentioned (at least with my Bottlehead Repro amp) also having an RIAA circuit is tougher.
Good thinking Ken.

[Kenkirk]

Thanks Steve,

I added a signature so you could see my set up. I use a Urishi that has .4mv output and it works well with the Cat, but is at the very limit of proper matching. If I change cartridges, I will try to find something with .5 or more.

If I buy the Seduction, I will build it myself as I enjoy kit building. I loved to go to the Heathkit store when I was a kid. By the time I was old enough to buy and build kits for myself, Heathkit was no longer. Bummer. 

I also am trying to figure out how to tap into the Otari with a switch so I can put it back to stock. I use it to mix down from my Tascam TSR 8 track. I also want to use it for some location recording. So I don't want it to be just a play back deck. It should be simple enough, but I would like to find someone who has done this before committing my mint Otari to surgery.

Just got my first tape yesterday. TP 001, Sweet!!!

[PJ]

...
Anyway, it got me thinking that maybe a little network could be made that reverses the RIAA and adds the IEC eq and maybe pads down the input some to match my phono stage on my Cat Ultimate preamp.
...

It's a clever idea. A quick look says this should be possible with a remaining treble rolloff at about 20kHz, so it would work pretty well - not quite perfect, but you'll also keep the 50Hz bass corner which is also not quite perfect. Both are very workable compromises though. I'll see what I can come up with, assuming a 50 k-ohm input impedance on the RIAA preamp, but it might take me a day or two.

[Kenkirk]

Excellent. I could even open up one of the reverse RIAA networks to see the values used. The tech used to work at Cat and knew what he was doing. I am sure Ken Stevens could put something on paper.

Ken

[Kenkirk]

I just remembered I also have an Old Dynaco Stereo tube integrated amp that has a tape head input. Maybe a schematic and some smart EE guy can come up with how to tweak the old dynaco for IEC.

Guess I am going to have to start splicing into the Otari soon. I just must have a tube head amp! :-)

Ken

[Ben]

I think would skip the dynaco as it was budget minded design.
I would look for  the 'top of the line' preamps out there for the early
60's valve products and build something new. PS I tend not to like
using 6DJ8's cause I have heard they have a high 3rd order harmonic.
--------
And when you get your design finished I'll have to steal if from you. :)

[ironbut]

Just got my first tape yesterday. TP 001, Sweet!!!

Hey Ken, if you like the tape, keep an eye out for one of her concerts. It'll blow you away!

I don't think you can go wrong with building and modifying a Seduction. They're dirt cheap and sound amazing for the money. PJ designed it and you'd have to spend at least 3 times as much to equal it I think. I had fun building mine and even bone stock, it has a very enjoyable sound.
Looking forward to seeing what's in your inverse RIAA. I have one in my Hagerman FryKleener Pro but you can buy just the inverse circuit from Jim.

[Kenkirk]

Yea, I need to just build a Seduction. Later when the catalog of 2 track tapes justify it, I would love to have the all out unit.

But I really love the Cat phono stage and it just might be possible to get excellent sound from it. I wish that tech at Cat still worked there. He could have whipped one up for me so quickly. He was really clever and knew the Cat inside and out.

Ken

[ironbut]

Yeah, that phono stage is killer! I had a SL1 Signature for 12 years before buying the Ultimate. I hate to think what some guys would charge for just the phono stage. I listened to the other full function preamps before upgrading,.. the Hovland was beautiful but the bass was absent, the Ayre K1x was a close second but it was a tad cold. One of these days I need to send the CAT back to Ken for upgrading (mines a Mk-1) but I'm so happy with it, I think it's the last thing in my system I'd ever really need to upgrade.
Heck,.. it even has a tape loop!

[steveidosound]

How is the Seduction as a phono stage for the price?
I have not constructed my kit which I have had for 3-4 months so far.
 I know it is more complicated switching, but has anyone managed to build one as RIAA phono, then another set of jacks and switch IEC and NAB for tape heads. Would putting that much stuff in there destroy the sound from the careful component placement and signal path length or make it have more noise etc? I don't want to destroy it's performance.

[docb]

You have to switch in different resistors and also an additional cap, so the switch setup would get a bit complicated. I am also hesitant to put in a switchable input when dealing with such low level sources and there could be an issue of source cable length depending on how the system is arranged. It can be done, but might be less than ideal in terms of noise levels and practicality. All that said, I haven't done it myself, and I try to avoid conjecture on this forum. Best bet would be to smoke it and see.

[PJ]

No such luck I'm afraid. Any RC passive corrector I can think of will lose at least 6dB of gain, be noisy due to high resistances, and still load the tape head with a low impedance at high frequencies.

The "natural" way would be an LRC series network, but inductors at high impedance are problematical to build and excessively sensitive to magnetic hum fields. I'm sure it could be done, but the custom inductors with triple or quadruple mumetal shield would wind up costing more than a Seduction even if you did not count the weeks of R&D to get it right.

[jcmusic]

While we are on this subject I have asked a few people about this idea. I have an Otari MX5050 BII-2 and was thinking about using the tape/source switch and wiring rca's directly to the playback head and the to a seduction. The switch according to the manual turns off the outputs when in source mode and without a signal from the imputs seems like this would work, if the head has enough output to the seduction. Does anyone have any knowledge or comments.

Jay

[Kenkirk]

Yeah, that phono stage is killer! I had a SL1 Signature for 12 years before buying the Ultimate. I hate to think what some guys would charge for just the phono stage. I listened to the other full function preamps before upgrading,.. the Hovland was beautiful but the bass was absent, the Ayre K1x was a close second but it was a tad cold. One of these days I need to send the CAT back to Ken for upgrading (mines a Mk-1) but I'm so happy with it, I think it's the last thing in my system I'd ever really need to upgrade.
Heck,.. it even has a tape loop!

Agreed. I have tried at least 4 or 5 high end preamps/ phono stages, but after several days of close listening, I always went back to the Cat Ultimate. I have not sent mine back for the Mark II upgrade either. I have 3 friends that all have the Mark II, and the only difference I can hear in their systems is a slightly lower noise floor. If mine needed work ( repair ) I would get it upgraded. Other than that, I not just looking for things to spend money on! Seen the stock market lately?? Ouch! :-)

I did pull out my reverse RIAA adapters. The guy closed them up with large pieces of heat shrink tubing. Looks like there are 2 caps, and they are probably those custom Cat caps with probably no markings. I need to find some large heat shrink before I rip them open. Anyway, the way he made them is there is a rca male on one end, a rca female on the other, and between them are a pair of caps, and several resistors all neatly wrapped up in heat shrink.

Ken

ken