Simple 5687-based preamp
Do you fancy a simple DIY project? So simple and so cost-effective that it gives you the most bang for the buck? And let's stay away from popular (and hence expensive) audio tubes like 12AX7, 12AU7...
Here's 2 very simple linestage preamps. The first credit goes to Audio Note. It's called the Audio Note M7 preamp. The second is a modification of it by KLANS resident DIY guru, Mr. vt4c. Here's some reasons why this project is so mojo.
1. A very linear and inexpensive tube, the 5687. Some people question the pedigree of this tube. They are nuts. You can question Audio Note's decision to use it in the quarter-million RM Ongaku or their best selling M7 preamp! : )
That's right. I love #4 as this project is a good/simple way for you to cut your teeth on DIY. Once you learn what works, and what doesn't, once you learn the cost-effectiveness of DIY, you can fly!
All right, which circuit?
Here is Audio Note's M7 preamp.
Here is vt4c's M7 preamp. There's actually a full write-up on The Star's DIY Den but there's no schematics there, so this should complement that page.
If you need the 5687, 6X4 tubes, sockets, capacitors and tagboard, you can get from me, from this page. If you need to build the power transformer, this vendor here can get one done for you at a very good price. I have no affiliation with this winder, so there's no middle-man charges here. Oh yeah, with end-bells as well.
Alternatively, you could also ask this vendor. I have more confidence with EKK as they flash test their transformers before handing over to you. Don't be suprised by this as almost all the local transformer winders I know of DO NOT run this test. They (the transformers, not the people) are ugly though as they don't come with end bells. Runs slightly hot but the voltages with load hasn't failed my expectations.
Sorry, I haven't tried other transformer winders. There's one I tried but opinion has been mixed, so I'll leave it as that.
A suitable power transformer specs will be 12.6V@2A, 6.3V@2A and 200-0-200V@100mA, for Audio Note's circuit. This is, of course, just a guide. You can definitely steer away from this. You could even use a single 6.3V@4A heater for both 5687 and 6X4 tubes though this isn't recommended.
vt4c's circuit specifies power trans of 350-0-350V@100mA, followed by 10H, 50uF, 10H, 50uF. Topology is shown below.
That's all to it! Simple enough? Think you can wire a transformer, a few resistors, few cap and 2 tubes correctly?
More information will be available on this page later.
Update on 6 January 2003:
I'm back! I'm more than back!
I built vt4c's circuit but with my own power supply. Here's the circuit:
I actually asked for the power transformer to be winded 350-0-350V but the winder got it wrong and gave me 330V only. Anyway, it still works out okay.
Lesson learned from this project : Trust me, you'll appreciate this later.
This little project lays the path to DIY a simple power amp to go with this preamp.
Okay, but thought you mentioned that this preamp will cost less than RM250 to build? Well, you COULD get it down that low. Try to minimize bulk in the power supply. Maybe you want to just use a simple CRC filter ala Audio Note's circuit. You save 2 chokes. Instead of using the Alps volume pot, you could build a cheap stepped attenuator ala Bottlehead's Sweet Whisper from this article. You could use Auricap 1uF 450V at RM32 each as well. There you go!
I built it with whatever I have on hand though I'll say the sound will change somewhat if you take out the bulk from the power supply. I like chokes and I'll stick that in. Your milage may vary...
Coming soon! Photos and step-by-step (almost) tutorial on how to build this simple 5687 preamp.
Update on Jan 9, 2003: It's ready! How to build your own tube preamp.
Update on Apr 6, 2003: Preamp update!.