John Broskie's Guide to Tube Circuit Analysis & Design

10 July 2011

Mono 66-Position Balanced Stepped Attenuator
Good news for the those audiophiles who run balanced systems. The BM-1 is a new stepped attenuator that I have designed. It uses 24 resistors and two rotary switches: one 11-position, 1-pole, shorting switch; one 6-position, 2-pole, shorting switch. The configuration is interesting, as it consists of a shunt stepped attenuator cascading into two series stepped attenuators, one for each phase.

As you can see from the schematic above, the BM-1 stepped attenuator presents five connections: two for the balanced input signal and two for the balanced output and one for the ground connection.

This is a very clever design, even if I do say so myself. Think about it: being able to set 66 -1dB volume decrements for a balanced signal source is a huge undertaking; and being able to do it with just 24 resistors and two single-deck rotary switches is a miracle. Imagine how large and crazy-expensive a two-deck, 66-position rotary switch and 132 resistors would cost, if a conventional series attenuator were used instead. (And a ladder stepped attenuator would cost twice as much as the series attenuator.) The small price we pay is a -3dB insertion loss, because of the shunt attenuator. Since most balanced signal sources already provide far to much voltage output, hence the need for a volume control, the -3dB loss is not a problem.

The extra-thick, 2oz copper PCB is 1.4 by 5.8 inches big and the rotary switch spacing is 3 inches. The PCB are made in the USA. BM-1 mono stepped attenuators include 8-page user guide with assembly instructions and a resistor-value tables.

The BM-1 balanced, mono attenuator is available now at the GlassWare Yahoo store and cost the same as the M1 mono unbalanced attenuator; dirty cheap for stellar performance, in other words.

http://glass-ware.stores.yahoo.net/

 

My Passive Line Selector and Stepped Attenuator Box
I was about to title this section "My Passive Line Stage," but this is a really a stage-less design. My latest project is made from two GlassWare M1 66-position stepped attenuators and one GlassWare Select-1 input selector and a black-painted Bud box (2.5 x 17 x 7 inches) and ten RCA jacks; this simple project uses no active devices, relying on me to provide the active knob turning. It can choose between three input signal sources, say a CD player, a tuner, and a phono preamp. Once a signal source is selected, each channel's volume can be set independently by the two mono stepped attenuators, each of which holds two knobs, one for fine (-1dB) decrements and one for coarse (-6dB) decrements. See Blog 204 for more details on this project.

I have yet to wire up the unit, but that task shouldn't take too long. (If I don't use Teflon-coated wire, it would only take half as long to assemble.) I am using carbon-film resistors throughout, as I want to avoid that harder sound that passive setups so often impart. The load impedance that the signal source sees is 20k, which is an excellent compromise, as it is not so low a resistance as to load down tube-based signal sources and not too high a resistance to drag down the high-frequency response due to cable and Miller-effect capacitance.

My eight year old son is keen to solder this project together. Since there are no finger-frying high voltages to worry about, I probably will let him. (I still need to add a ground post and a toggle switch for selecting grounding options: no ground, just floating the chassis; house ground, via a wire to the third prong socket on the wall socket; and signal ground, via to 10-ohm resistors to each channel ground.)

The interesting thing is that I do not actually need another passive switch/attenuator box, as I already own several. But I find this project so coherent and elegant that I cannot resist building it. Before anyone is tempted to ask, yes a balanced version could easily be made by using two BM-1 mono attenuators and the same Select-1 signal selector switch. (In fact, I will probably build one, but with no selector switch; instead, I will use my phase switch and PCB.)

 

 

Next Time
I will be back in more ways than one. I plan on getting back to the topic of hybrid tube Op-Amps and return to the topic of Moskido hybrid power amplifiers.

//JRB

     

Kit User Guide PDFs
Click image to download

BCF User Guide

Download PS-3 User Guide

SRPP+ User Guide

Aikido All in One user guide
Janus regulator user guide

Aikido Phon Preamp user guide
Aikido 9-Pin Stereo User Guide

E-mail from GlassWare Customers

Hi John,

I received the Aikido PCB today - thank you for the first rate shipping
speed.

Wanted to let you know that this is simply the best PCB I have had in my hands, bar none. The quality is fabulous, and your documentation is superb. I know you do this because you love audio, but I think your price of $39 is a bit of a giveaway! I'm sure you could charge double and still have happy customers.

Looking forward to building the Aikido, will send some comments when I'm done!

Thank you, regards,
Gary.

And

Mr Broskie,

I bought an Aikido stereo linestage kit from you some days ago, and I received it just this Monday. I have a few things to say about it. Firstly, I'm extremely impressed at the quality of what I've been sent. In fact, this is the highest quality kit I've seen anywhere, of anything. I have no idea how you managed to fit all this stuff in under what I paid for it. Second, your shipping was lightning-quick. Just more satisfaction in the bag, there. I wish everyone did business like you.

Sean H.


High-quality, double-sided, extra thick, 2-oz traces, plated-through holes, dual sets of resistor pads and pads for two coupling capacitors. Stereo and mono, octal and 9-pin printed circuit boards available.

   Designed by John Broskie & Made in USA

Aikido PCBs for as little as $24

http://glass-ware.stores.yahoo.net/

 



Only $9.95
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TCJ Filter Design

The Tube CAD Journal's first companion program, TCJ Filter Design lets you design a filter or crossover (passive, OpAmp or tube) without having to check out thick textbooks from the library and without having to breakout the scientific calculator. This program's goal is to provide a quick and easy display not only of the frequency response, but also of the resistor and capacitor values for a passive and active filters and crossovers.

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While the program’s main concern is active filters, solid-state and tube, it also does passive filters. In fact, it can be used to calculate passive crossovers for use with speakers by entering 8 ohms as the terminating resistance. Click on the image below to see the full screen capture.

Tube crossovers are a major part of this program; both buffered and un-buffered tube based filters along with mono-polar and bipolar power supply topologies are covered. Available on a CD-ROM and a downloadable version (4 Megabytes).

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