Ebay Attenuators

[Bottlehead]

I just made an impulse buy on ebay--the seller has several different attenuators listed, this is a 24-position unit using all Vishay-Dale 1-percent resistors for $52. I'm no expert, as far as I can figure it's a stereo series attenuator with a constant impedance of 100K. Not being attenuator-savvy I may be wrong, perhaps it's a ladder L-pad. Either that or it's a fixed-series variable shunt. It's sure prettier than an ALPS or Noble doohickey for about the same price, if nothing else. Questions for those who know: Since it kind of defeats the purpose of gold posts, pricey interconnects etc. to feed an audio signal through 20 yards of cheap resistance wire, what type of stepped attenuator is best for A) direct control of a power amp like Baby Blue and B) controlling the volume of a DIY preamp (I'm trying to decide what to do with my dyna PAS3, it doesn't cut it in stock configuration).

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... RK:MEWA:IT

[EWBrown]

A few months ago, I bought an earlier version of this stepped attenuator and it was very nicely made. This one was assembled with the resistors sticking out radially, so the total diameter is a little over 3" but the depth is about half of the unit depicted in the auction.

Instead of a series string of 23 resistors that add up to approximately 100K, as is done in the smaller and cheaper "Alps" units from another Taiwan seller, this one has 2 sets of 23 pairs of resistors that each add up to approximately 100K, and give approximately 3 dB steps. The switch is a 24 position, 4 pole, one position is a "stop" , so that the swtch doesn't just turn continuously around.

Just to buy the switch and the 92 resistors here in the US would cost far more than the $52, not to mention the hours of hand soldering that would be required to put it all together.

It is indeed a bargain, and I'm really satisfied with the one I bought.

/ed B in NH

[DortoH]

Very nice looking attenuator.

I was thinking about ordering a Sweet Whispers kit from Bottlehead.com but prefer the stereo attenuator over a dual mono setup.

Thanks for the link, I think I'll check them out :)

Rod

[Bottlehead]

There several types of attenuator designs and I'd like to know what's best in specific circuits. I design speaker crossovers and in the event the finished product isn't perfect (which hardly EVER happens, honest) I install temporary adjustable L-Pads in driver circuits to dial things in before selecting hard-wired resistors. Input impedance remains constant, meaning I'm not affecting other crossover components while increasing or decreasing tweeter or midrange dB output (theoretically, with fingers crossed).

For use in stereo amp circuits, here are the three basic types I'm aware of:

-- Ladder L-Pads, with switchable series and shunt resistors for each position so the audio signal only passes through a single series element no matter how many dB of attenuation. Input impedance is constant (the load stays the same) while output impedance varies.

--Fixed series/variable shunt, using a fixed single resistor for the series connection while the switch controls the shunt resistors. Input impedance varies with every setting. This seems (to me) like a stepped pot rather than an actual attenuator, with resistors replacing the nichrome resistance wire.

--Series attenuators, where the signal passes through a string of resistors which add up to the impedance of the control no matter what degree of attenuation is used. Input and output impedance both remain equal, as I understand it. The only drawback, if you're an audiophiliac, is that the signal passes through several resistors instead of one.

Feel free to correct me if I'm wrong, as I've said before I'm electron-challenged and a dyslexic schematic reader.

Anyhow, back to my original question. For direct gain control of Baby Blue, which type is best? If I replace the pot on my modded PAS-3 with a stepped attenuator, which type should I use?

Thanks for all input, and I hope I steered some of you to a bargain deal with the ebay link--it certainly appears to be much higher quality than standard ALPS or Noble $40 attenuators and looks awfully sexy to boot.

[Bottlehead]

I have several power amp projects where I'd like to use quality attenuators and I considered buying more than one of the Vishay Dale ebay units. Then a light bulb illuminated and I realized it wasn't necessary, why not put one in a box with RCA connectors and build a passive pre I could use however I choose. I've never taken on a passive pre project before and did a bit of web searching--most of the designs I came across use 20K attenuators. Are there any designs out there using a 100K pot? It seems that it'd offer greater flexibility. Say I wanted to connect a CD or DVD player through an external attenuator box directly to a Dynaco power amp--Baby Blue, an ST70, a pair of MkIII's--I'm going to have impedance-matching problems. How is this best dealt with? Am I better off with a simple active pre, if so can someone recommend a circuit?

[EWBrown]

My homebuilt "ultimate simple passive preamp" is simply a small aluminum box, a 100K stereo "Alps" pot sold by Rad Shaq for $2.99 and 4 RCA conectors (I didn't bother with fancy gold plated isolated ones here, just the basic ones which were some leftovers from a Mouserr parts order - the same ones spec'd in the original reV B parts list), Input goes across the outer pair(s) of the pot so it presents 100K to the source, and the siper and ground go to the outputs. I use it with any of my amps if they don't have their own volume pot or attenuator, and if the source is a fixed line level output - as most cheap CD players are equipped.

The next step up is to use a stepped attenuator like eother the fancy one shown in the original post, or the cheaper ($15) ALPs "knockoff" shown next to it. Note that the E-bay $15 "Steeped Attenuators" from Taiwan are most likely Chinese-made copies, not the real ALPS, but still a decent deal for the price. Goldenears may not agree, but it's goodenough for me... :lol:

Next step, if gain is needed, is to go to a simple linestage, and there are many designs here, each has its merits and downsides, so I won't reccomend any one design over another here. Discussions of linestage designs can become as heated as discussions of comparative religion, so I'll stay neutral in this case.

I haven't yet tried it, but since I have a couple extra PAS2/3 preamps found for cheap/free, it may be a good expirement to remove all the tone control circuitry and "filters" just leaving the volume, and perhaps balance controls, in a Dyna PAS series preamp, and change the 12AX7s in the "linestage" portion to a lower gain 12AU7 or other lower mu pin compatible double triodes. Save those Telephunkin' 12AX7s for future usage as inputs for DIY-35s 8) Note: in the PAS presmps, the board under the selector switch is the phono and tape head gain stage, everything else bypasses this and goes straight to the tone controls and "linestage board" through the switch.

The 10K-25K attenuators are best for solid state use, 100K to 1 Meg are best for tube use, the "fixed" resistance goes to the source, the variable
"wiper" goes to the input grid, either directly or thru a cap, user's preference. I typically run with 100K, except in teh "stainless steel behemoth ST-70 Dynaclone in which I used a 2X 500K pot, as it approximated the original 470K input resistors of the Dyna ST-70. THis was done in order to keep it 100% compatible with Dynaco PAS3 preamps and FM-3 tuners. otherwise 100K is fine.

In an SCA-35 to ST-35 conversion discussed elsewhere, I used one of the ALPS 100K "steeped attenuators" as the volume pot, and dispensed with all the rest of the preamp and tone control sound-ruining circuitry altogether. No balance, no loudness, no filters, nothing to otherwise "modify" the source audio.

I call them "steeped atteuators" because that is exactly how the seller listed them on E-bay, if he hasn't corrected two gooder speling, then this can be used to search them out.

/ed B in NH

[Vince]

Mr. Brown,
Thanks for posting the details of your "Pre". It is going to solve my problems.
The local RS only had a 100K stereo pot. Is it mandatory that for an ST70,
using the VTA board, that it be 500K? Thank you, Vince

[Vince]

Hi Mr. Brown,
What you said above, with regard to the 470 ohm input impedance requirement of the ST70 which the PAS(X) provides , makes me wonder how universal, and useful, the PAS is. Can it be used with other amps or will it be restricted to the ST70?
Dumb time to be asking this question as I already have the VTA kit to modify my PAS.
Thanks, Vince

[EWBrown]

The 470K ohms input is not all that critical, it's basic function in life is to be the grid resistor for G1 of the input tube, be it 7199, EF86, 12AX7, 12AT7, etc. The input resistor could be anything from 100K to 470K,
typically use 100K and 270K. In days past, it was the common practice to use higher input resistance, 470K and 1 megohm were pretty much "standard".

The VTA driver boards are modelled after VTL's (Vacuum Tube Logic) driver circuitry, and they use either 270K or 135K (2 x 270K in parallel) for the input / grid resistor. For home use, 270K is the better choice, 135K is intended for studio / professional usage, as it matches the rest of the studio equipment.

The output of the Dynaco PAS series preamps is what is considered as being "high impedance" ,and should work with just about any tube amp.

Most SS amps, however, tend to have much lower (10K to 50K) input impedances, so the mis-match could cause problems.


For a volume control / attenuator, I normally use 100K, anything from the Radio Shack stereo pot to the "Hong Kong" 100K stereo stepped (ladder) attenuators found on E-bay..


These attenuators are built in two form factors, I call them "long" and "radial".

The "long" ones are about inches overall length, and the resistors are laid down against the switch body, which keeps the overal diameter to around 1.5 inches.

The "radial" ones use a shorter length switch, and the resistors extend radially outwards, and the "free" ends are all grounded to a ring of heavy buss wire. Their overall diameter is around 3 to 3 1/2 inches.

The actual circuit and function are identical, just the form factors are different.


I've also made a passive pre with a 100K pot in a box, and another using two of the "sweetest whispers" 15K, 11-step attenuators from Doc Bottlehead (these were leftovers from a "foreplay III" linestage, in which I used one of the stepped attenuators, instead). it works well with an SS source (normally a cheap CD player) into the 15K side, and then the step / "wiper" feeds the amp under test. 15K would be too low for most tube sources, unless ther final stage has a cathode follower (as does the "Foreplay" series). Eleven steps may seem to be too "coarse" but it works out very well, and I've never felt the need for any smaller "in-between" steps.

The Hong Kong lader attenuators are vrey well made, using Vishay / Dale 1% resistors, and a decent quality 4 pole, 23 or 24 position rotary switch.

They may not quite be "Goldpoints" but then they cost a fraction of the price, and I've always had good results with them. Good quality part,s and relatively low cost labor from Asia.


/ed B in NH

[Ty_Bower]

Would you guess this one is any good? He doesn't say if it is a ladder style or serial type. I'd guess the ladder type is preferred?
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0164950156

This one says ladder. It's hard to see, but there's a second ring of resistors tucked in underneath the ones that are clearly visible.
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0163541405

These come in 100k and 250k versions:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0171404688
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0171299617

[EWBrown]

The first one looks to be a "serial" attenuator, which is basically basically a "tapped pot". Someone had to have good eyes and a very steady hand to do that soldering job :o Yellow_Light_Colorz_PDT_11

The others are "ladder" type, they will have two resistors per step, which add up closely to the specified resistance, and each step is a two-resistor voltage divider.

The one with the small surface mount resistors between contacts appears to be "serial", as are the small blue "alps" attenuators. THose are "knockoff" copies of the real ALPS attenuators, I bought a couple several years ago, upon inspection, they have 23 tiny resistors in series, for each of the two channels. They are not the "real" ALPS pots, but for most purposes they are more than good enough. I used one in an ST-35 modded SCA-35, the other in a HB EL34 SE amp. For those, they worked very well.

/ed B in NH