Tube Buffered LM1875

[r0n22]

Hello all,

I am thinking that I am going to make a Tube buffered LM1875.

The reason why I am thinking a LM1875 is because I already have 2 transformers that are 16vac which would work nicely for the dual supply for the LM1875.

I want to get my feet wet with tubes and am in the process of building a tube amp but I dont have the money right now. So I want to build a Tube front end onto this chipamp.

I have not seen much on the web about tube buffered chipamps and the website's do not have alot of information.

I have found this one and thisas well as this one .

I was wondering what would be a good front end to the amp?

I have liked the Tube Cad Aikido Amplifier but I do not know much.

I am kind of at a lost to how to create a hybrid like this. Any good insight would be greatly appreciated!

[planet10]

Read thru Joe's tube buffered gainclone thread... that's the way i'd go (with a CCS on the tail of the cathode follower.

dave

[ChrisA]

The problem is that the tube wants to run on a high voltage B+ supply and needs either 6V or 12V for the heater. You can't use anything from the chip amp power supply so you need an independant PS for the tube(s). One you do that you may as well build a stand alone tube based pre-amp.

A tube pre is a good project becasue you don't need expensive transformers. No output transformer is required at all and the power transformer only needs to put out milliamps.

the common 12AX7 or 12AU7 is easy to get as they are current production items

I would __strongly__ suggest using conventional tube construction techniques. A mettle chassis with tubes and transformers on top and point to point wres nder. Do NOT try and mix the chip amp and tube in the same chassis. the reason is that a tube's B+ voltage is lethal. Keep that high voltage stuff away from everything else.

[tomchr]

I'd recommend that you adjust your approach to your skill level and the level of danger that you are comfortable with.

If this is one of your first projects, I would suggest reading up on tube technology (Tube forum, Morgan Jones "Valve Amplifiers", Valve Wizard, etc.) Also study electrical safety. Then you can start building tube circuits.

If this is one of your first projects, I would suggest separating it into two - preamp, power amp - simply to limit the number of headaches. But once you have completed both projects there's really no reason why they can't reside in the same chassis. I don't see any clear advantage of this, but also no reason against it.

The tube amp will likely need relatively high voltages. +150~250 V at a few 10's of mA seems pretty reasonable for a preamp. Plus you'll need either 12.6 V or 6.3 V at upwards of 1 A for the heaters. As some of the heaters may be floating at higher voltages, I wouldn't plan on deriving any power for the heaters from the power amp supply.

Note that all components have a maximum operating voltage rating. Even resistors (typ 250 V for leaded parts, less for SMD) and wire (typ 300 V). If you stay below 250 V, you usually don't need special components. But pay attention!

Good luck.

~Tom

[becki200]

Hi i would suggest to build the LM 1875 and buy a tube buffer from arjen helder on ebay.
This tube buffer works with a 12 Volt power supply. Makes things easier and safer. Best regards Thorsten

[ChrisA]

Originally Posted by tomchr

... there's really no reason why they can't reside in the same chassis. I don't see any clear advantage of this, but also no reason against it...

There is no need to keep them separated as long as everything works and keeps working. But you have to think about failures and what if a wire comes loose. The other thing is that tube gear is VERY sensitive to layout and I'd not want a high current wire near a wire that was headed to a tube's grid.

But you are right that in theory there is no reason, it's more a practical reason.

[tomchr]

Originally Posted by ChrisA

There is no need to keep them separated as long as everything works and keeps working. But you have to think about failures and what if a wire comes loose.

That's really no different from a solid state amp or chipamp. People seem to forget that just because the secondary side of a transformer provides low voltage that the entire design is low voltage. The primary is still connected to the mains (high voltage). And in some cases - like (>50 W) power amps - you also have high voltage on the secondary side.

Originally Posted by ChrisA

The other thing is that tube gear is VERY sensitive to layout and I'd not want a high current wire near a wire that was headed to a tube's grid.

You can say that about any amp. I have achieved 30+ dB of improvement in the hum and noise performance of a chipamp by routing the ground connections properly (star ground). The nodes around the input and Vbe multiplier are quite high impedance, thus very sensitive to layout. My tube amps have all been built on a prototype board that I set up. They've performed well in that setup. Of course, the hum and noise will improve with better layout, but it's not like it's going to blow up in your face. You can see examples of my DeathTrap layouts here and here. Both designs worked in this setup without signs of instability.

~Tom