Recommendations for a mid-priced tube preamp

[opnly bafld]

It just seems like a very odd design choice on their part.

A low output impedance tube preamp will work well with any amp, even SS or Class D. Since so much of the interest in tube preamps seems to be from people who want to add some "tube sound" to their SS and Class D based systems, you would think Frenzel would be building something that's suited to that market. I suspect that most tube amps don't even have an input impedance over 100k.

Agree and why I said earlier since they do market it as a stereo hi-fi preamp.

I currently have 3 tube amps, 1 well over 100k (60? years young), 1 @ 100k (less than 1 y.o.), and 1 well below 100k (17 y. o.).

 

[Tinkerbelle]

It just seems like a very odd design choice on their part.

A low output impedance tube preamp will work well with any amp, even SS or Class D. Since so much of the interest in tube preamps seems to be from people who want to add some "tube sound" to their SS and Class D based systems, you would think Frenzel would be building something that's suited to that market. I suspect that most tube amps don't even have an input impedance over 100k.

Their website says it's the same preamp used in their integrated amps, so perhaps the preamp was a follow-on from those rather than initially intended for wide market use. Basically, perhaps just using that existing design and making it a standalone item, possibly also used for their power amps?

 

[ceylon]

I tried a Xiangsheng 728A (wrote it up at
https://audiokarma.org/forums/index...rt-xiangsheng-728a-tube-pre-amplifier.963788/ )
Pleasant, but you could tell there was a piece of equipment in the chain. Ideally, components should be "invisible," no?

It was better than an Emotiva BasX PT1, which I quickly returned on the 30-day trial policy.

But far above both is the Van Alstine Transcendence 10 RB. Natural, full, none of the over-brightness of so much SS. The price is almost twice your ceiling. That was more or less my ceiling, too, but I am glad I went for it. I expect it to last a long time, so I excuse the cost by spreading it out over the years.

theres a silver-faced 728A return u can purchase at china hifo for $180 right now!! $180!!! and OP wants to hear some
color in that signal chain

 

[lattiboy]

I

theres a silver-faced 728A return u can purchase at china hifo for $180 right now!! $180!!! and OP wants to hear some
color in that signal chain

Unfortunately shipping is $180 additional dollars on that refurb. Think I’ll keep my eyes peeled for a domestic unit.

 

[Stephen300]

I just recently finished assembling a 3-input 4S Universal Preamp using a 12AU7 and an EZ81 rectifier and I couldn't be happier with the results. This is a low parts count project that sounds great and is absolutely dead quiet in operation. The only thing I added to the design was LED indicators for which input source is active. I did so by using a 4-pole 3-throw rotary switch for the input selector. If you're into DIY, and are familiar with working with high voltages, you might want to give it a try.

 

[mcgjohn]

@lattiboy , I have owned two copies of the 728A. They are ok, but if your budget is 500, the Marantz clone is substantially better.

 

[ceylon]

I


Unfortunately shipping is $180 additional dollars on that refurb. Think I’ll keep my eyes peeled for a domestic unit.

oh wow really? i sure didnt expect that.
heard the marantz 7 phono clone is even better than the ear 834p clone (black board gold traces).

 

[PakProtector]

You can build an active load for a triode that delivers output Z in the under 100R range easily. Last one I measured was 30 Ohms at 20 mA idle current. 6AH4 or 12B4 turns in moderate gain...and fabulous sound. Tube rectifier, choke input filter, and motor run caps with a 120V>heater switch-mode supply to give easy DC to the heaters...hubba-hubba. 250V of B+ at most.
cheers,
Douglas

 

[CharlieAudio]

Unfortunately shipping is $180 additional dollars on that refurb. Think I’ll keep my eyes peeled for a domestic unit.

I had to return mine for fixup work. It went to a U.S. address, apparently a contracted repair station. So it is likely that $180 shipping to buy a refurb has nothing to do with China-U.S. transit.

 

[DAnny45acp]

I just recently finished assembling a 3-input 4S Universal Preamp using a 12AU7 and an EZ81 rectifier and I couldn't be happier with the results. This is a low parts count project that sounds great and is absolutely dead quiet in operation. The only thing I added to the design was LED indicators for which input source is active. I did so by using a 4-pole 3-throw rotary switch for the input selector. If you're into DIY, and are familiar with working with high voltages, you might want to give it a try.

I'll second that!! Been using that little guy a a couple years now - driving my Dynaco ST-70 - it really defies logic in it's design - even with the volume pot on it's output but dam it reallys sounds fantasitc! Very simple and only a small handful of parts- the build is so easy a caveman can build it.

 

[Lavane]

I'll second that!! Been using that little guy a a couple years now - driving my Dynaco ST-70 - it really defies logic in it's design - even with the volume pot on it's output but dam it reallys sounds fantasitc! Very simple and only a small handful of parts- the build is so easy a caveman can build it.

Yup. I built one awhile back and it's connected to my ST-70 in a second system at the moment.
The 12AU7 and 12AY7 work the best with a slight nod going t9 the 12AY7.
Did you add the optional 33UF caps across the cathode resistors? I put them on a switch. It sounds better with them switched in at lower vhmes, and swit he'd out at higher volumes.

 

[DAnny45acp]

Looks good, no I don't have the 33uF bypass installed.. great idea with a switch to add or remove in the ckct.. I'll have to add that and give it a try. I built mine on a very small 4x8 hammond aluminum chassis- I also use a nice 6ft ac powercord wired directly in with some nice strain relief connectors I have- for me it's too much work cutting and filing to install those AC connectors - just one hit with the drill bit for the cord and another hit for the fuse holder - done! I assume 1 switch you have is for power the other for your 33uF .. what's the 3rd switch for?

 

[Lavane]

View attachment 2673506 Looks good, no I don't have the 33uF bypass installed.. great idea with a switch to add or remove in the ckct.. I'll have to add that and give it a try. I built mine on a very small 4x8 hammond aluminum chassis- I also use a nice 6ft ac powercord wired directly in with some nice strain relief connectors I have- for me it's too much work cutting and filing to install those AC connectors - just one hit with the drill bit for the cord and another hit for the fuse holder - done! I assume 1 switch you have is for power the other for your 33uF .. what's the 3rd switch for?

The third switch is for the optional ground lift. Don't bother with that one.
The 33uf cap adds a bit of heft to the low end at low volumes, but tends to be a bit much at higher volumes. It's sorta like a loudness tap, but overwhelming at higher volumes. What tube did you go with? I found the 12AX7 to have way too much gain. The 12AY7 works best for me with the 12AU7 close behind. I used an aluminum plate. I keep a few on hand for builds.

 

[DAnny45acp]

Ok now I remember .. he had that ground lift switch .. I never had any hum and never installed it. I've been using the lowest gain tube the 12AU7 .. I just looked at his chart again- Iam going to try and dig up a 12AV7 to try .. on his chart it shows lowest distortion. I'll have to give the 12AY7 a try also .. as far as I remember I only tried the 12AU7 as it's been more than enough gain for me.

 

[Lavane]

Ok now I remember .. he had that ground lift switch .. I never had any hum and never installed it. I've been using the lowest gain tube the 12AU7 .. I just looked at his chart again- Iam going to try and dig up a 12AV7 to try .. on his chart it shows lowest distortion. I'll have to give the 12AY7 a try also .. as far as I remember I only tried the 12AU7 as it's been more than enough gain for me.
View attachment 2673521

I've never had a 12AV7. I'll have to keep a look out for one.

 

[Stephen300]

Built my 4S Uni-Pre without the 33uf cap and ground lift switch. The thing works so well I just haven't felt the need for either of them.

 

[Lavane]

Built my 4S Uni-Pre without the 33uf cap and ground lift switch. The thing works so well I just haven't felt the need for either of them.

The ground lift isn't needed at all. I've used this preamp with various amps without any problems with noise. The 33uf caps did flesh out the low end and lower midrange at lower volumes. It's cheap and easy to try it. I just used a couple Panisonic lytics.

 

[Stephen300]

Let's say I add the 33uf bypass caps and place them on a switch. When the switch is actuated what risk is there of it causing a loud POP thru the speakers? Will placing the switch on the positive or negative side of the caps make any difference?

 

[Lavane]

Let's say I add the 33uf bypass caps and place them on a switch. When the switch is actuated what risk is there of it causing a loud POP thru the speakers? Will placing the switch on the positive or negative side of the caps make any difference?

I just put the switch on the negative side of the cap. I dought it would make a difference though. I haven't noticed any pop when using the switch.

 

[Stephen300]

When I get around to it I may just give it a try.