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    passive pre amp?

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    pscottlowe

    Posts : 37
    Join date : 2011-09-04

    passive pre amp?

    Post by pscottlowe on Thu Feb 02, 2012 2:21 pm

    I have been using a pas-3 and an st-70 to drive my LaScalla.I have been using only one source:A Denon cd player.I read somewhere about a passive pre amp:described as "just a box with two attenuators ,and it was all that is between the source and the amp.I am very interested in trying one of these as i have been having channel imbalance issues .I have trouble shot it as in the pre amp stage.any guidance would be very helpfull . as i said i am only using one source. what about the attenuator option on the new kits would that work on my old st-70 ? and if so how difficult is that to install?I have 3 amps i would like to try it out on all of them so could that option be installed in "a small box" and function as if it was installed on the amplifier I still want to get the st-120 but i want to sort out the irritating imbalance problem that i have with my pre amp prior to that upgrade.

    Tube Nube

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    Age : 53
    Location : Calgary, AB

    Re: passive pre amp?

    Post by Tube Nube on Thu Feb 02, 2012 3:36 pm

    Hi Scott,

    First thing I'd do, were I you, is confirm it's the pre-amp. I'd reverse the channel leads from the CD player, just to confirm it's not the source.

    Next, I'd reverse the leads from the pre- to the power . . . to confirm it's the Pre, and not the power amp.

    Next, I'd (after turning everything off, and letting it cool down) remove and re-seat all the tubes, without changing their locations.

    Next, I'd change up the position of tubes from right to left channel -- one set of tubes per test, in order determine if it's a tube, whilst isolating the culprit.

    Failing that, I'd . . . well, I'd post here and hope someone who knows more than me would pitch in.

    ;-)


    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: passive pre amp?

    Post by tubes4hifi on Thu Feb 02, 2012 3:46 pm

    I get over 3 million links google-ing (how do you spell that?!) so plenty out there to choose from!
    All you need is a box, a selector switch, and however many RCA jacks you want. Simple!
    You can buy ready made ones for anywhere from $50 to $1000.
    Here's a nice little box for $99 plus $7 shipping!! http://diyhifisupply.com/node/196

    (this post moved from the "for sale" sticky, where it didn't belong)

    Bob Latino
    Admin

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    Re: passive pre amp?

    Post by Bob Latino on Thu Feb 02, 2012 5:57 pm

    Hi Scott,

    The VTA stepped attenuator can be installed in any ST-70 (original or VTA ST-70 kit) or ST-120. It can also be adapted for any "basic" stereo amp that has no volume control as long as you are willing to drill a 1/4 hole. On the Dynaco VTA type amps it just fits into the old stereo/mono slot so no drilling is needed on any ST-70/ST-120 amp. Using the VTA stepped attenuator gives you (in effect) a single source high level "integrated" amp. If you have more than one signal source and want to switch between them, then you need a preamp.

    The stepped attenuator only has 6 solder connections. It takes about 1/2 hour to install and is not that difficult to do. You just remove the stereo/mono, install the attenuator in its place and wire up the attenuator.

    Bob

    j4570

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    Join date : 2010-08-30

    Re: passive pre amp?

    Post by j4570 on Thu Feb 02, 2012 8:49 pm

    I installed the attenuator when I rebuilt my ST70 and brought it new life. I like it a lot.

    But troubleshoot the real problem first. Also, some CD players have variable output, like my ADCOM. You don't need a preamp that way either.

    pscottlowe

    Posts : 37
    Join date : 2011-09-04

    Re: passive pre amp?

    Post by pscottlowe on Sun Feb 05, 2012 3:39 pm

    OK so i definitely traced it to my pre amp . I would like to use the stepped attenuator to build a passive pre amp that i can use with any of my amplifiers [ 2) st-70's ,1) adcom gfa 555 II,and a yamaha .I have only used one source for months and want a very simple attenuator that i can apply to any of the above.So is this a practical idea? and if so could i get some information on how to best do this if its possible .

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: passive pre amp?

    Post by kaner on Mon Feb 06, 2012 8:36 pm

    I built a pair of stepped attenuators that I use as a passive pre. Sounds great and it was not all that hard to build. I spent a great deal of time worrying about the impedance balancing (do a search in this forum for my posts). Anyway, if you can build an ST70 you will have no problem with the attenuator. Let me know if you're interested in my resistor selections and I will email the spreadsheet to you.

    tubes4hifi
    Admin

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    Re: passive pre amp?

    Post by tubes4hifi on Mon Feb 06, 2012 9:15 pm

    if you want to spend an hour getting parts and another hour wiring them together, that's fine, but IMHO I'd rather just buy the stepped attenuator Bob has for $50 and be done with it in 2 minutes!!

    mantha3

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    Join date : 2010-11-10

    Re: passive pre amp?

    Post by mantha3 on Tue Feb 07, 2012 5:25 pm

    Check the input impedance on the amps... The ST120 I think has 180,000 input ohms which is really easy to drive and the passive works great.. Some amps (solid state in particular) have in input impedance of 20,000 input ohms so the passive my have trouble driving it.

    The stepped attenuator Bob has is excellent and I run my ST120 with it. Should you go with a preamp later you can just turn it to 21 out of 21 for bypass mode. Nice.

    pscottlowe

    Posts : 37
    Join date : 2011-09-04

    Re: passive pre amp?

    Post by pscottlowe on Tue Feb 07, 2012 7:25 pm

    thank you ,I have an ST-120 on its way with the stepped attenuator option( should get here on 2-13-2012) I will report back my findings! If its still out of balance I will BE shopping for a CD player. I would love input on a suitable cd player .The entire system would likely benefit from a cd player as the one i am using is about due for replacement .(bought it at goodwill 3 years ago)SYSTEM: klipsch LaScala, Dynaco vta st-120 with all upgrades and a low gain board.Thanx for all the excellent advice ,Scott

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: passive pre amp?

    Post by mantha3 on Tue Feb 07, 2012 7:34 pm

    I hope ya got the Capacitor upgrades too. I bet ya did.

    frank

    Posts : 95
    Join date : 2011-02-14
    Location : Pacific Northwest

    Re: passive pre amp?

    Post by frank on Tue Feb 07, 2012 11:39 pm

    I am a big fan of passive pre-amplification. If you have enough gain; that is the way to go - in my opinion. Just make sure to use the shortest possible interconnects (no 10 ft connections).
    You will notice improved clarity\transparency; with only the drawback of lower volume.

    pscottlowe

    Posts : 37
    Join date : 2011-09-04

    ST-120 volume problems?

    Post by pscottlowe on Fri Feb 17, 2012 2:54 pm

    It must be the additional power because the st-120 with the attenuator has zero volume problems and its perfectly balanced (left to right) I am using the amp sitting on top off the CD player with 12" rca cables and a few 90 degree adaptors and 8 foot speaker wires into klipsch LaScala and a JBL 2245h 18" sub driven with a 500 watt plate amp and wow, no volume or imbalance issues .Nice ,very nice amp.

    Bob Latino
    Admin

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    Re: passive pre amp?

    Post by Bob Latino on Fri Feb 17, 2012 4:07 pm

    fswidecki wrote: I am a big fan of passive pre-amplification. If you have enough gain; that is the way to go - in my opinion. Just make sure to use the shortest possible interconnects (no 10 ft connections).
    You will notice improved clarity\transparency; with only the drawback of lower volume.

    I do not understand what you mean by "the only drawback is lower volume" ? The VTA ST-70/ST-120 amps only need about one volt of input for full output. A CD player has about TWO volts output. Way more voltage than the either amp needs for full output. The stepped attenuator only "reduces" the high level input's voltage to a level that is acceptable to your ears. There is no "lower volume" associated with the VTA stepped attenuator. What passive preamp are you referring to that has "lower volume"?

    Bob

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: passive pre amp?

    Post by mantha3 on Fri Feb 17, 2012 7:39 pm

    I have the ST120 with the attenuator... Not sure what the "lower volume" comment is.

    One thing that is nice... Should you need longer interconnects or feel some dynamics are missing... Read up on "Buffers". A simple buffer stage running from the source cna be a nice touch. A fun simple project build

    GP49

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    Re: passive pre amp?

    Post by GP49 on Fri Feb 17, 2012 8:16 pm

    "Lower volume than a preamp with gain" is probably what they mean.

    Something like the old saw about, "I only have to turn my volume control 1/25 of the way up to play real loud." That only
    makes a difference on the rare occasion when you have a source, record, CD or software at VERY low level; the rest of
    the time it's a nuisance because it makes fine volume adjustments more touchy and difficult.




    pscottlowe

    Posts : 37
    Join date : 2011-09-04

    Re: passive pre amp?

    Post by pscottlowe on Sat Feb 18, 2012 10:19 am

    the ST-120 with the attenuator is hands down the best I have ever owned and i like the LESS equipment of no pre amp so I do not know what is the "volume problem" Mine is as loud and clear as any I have ever heard.I have literally been searching for this exact sound for years .This is an amazing value .So something else is wrong IMHO and this is a good place to figure that out because these guys have been an ideal resource to help with problems with the other part of my system and as soon as i employed my st-120 with no pre amp I was and am blown away by how good it sounds : both super clear and yes if you want it super loud .Something else must be wrong with the rest of your system

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: passive pre amp?

    Post by dynacoman on Mon Feb 20, 2012 12:53 am

    I'm currently building a lightspeed passive pre amp to try out. Should be sweet!

    frank

    Posts : 95
    Join date : 2011-02-14
    Location : Pacific Northwest

    Re: passive pre amp?

    Post by frank on Tue Feb 21, 2012 10:46 pm

    GP49 wrote:"Lower volume than a preamp with gain" is probably what they mean.




    Yeah, what he said.

    Or, an ST-70 with the low-gain driver board. Throw in a low-output source and inefficient speakers, a large room and a desire for loud levels and there will be disappointment.

    Sorry, for the confusion. I see I should have directed my comments more toward pscottlowe than a generic reader.

    'Frank

    pscottlowe

    Posts : 37
    Join date : 2011-09-04

    Re: passive pre amp?

    Post by pscottlowe on Wed Feb 22, 2012 1:55 pm

    Yeah, thats got to be it : I am using the low gain in an ST-120 with a DENON DCD-1000,and klipsch LaScala speakers that are aprox 9' apart and about 9' from my listening position . and volume is not the least bit of an issue! the only volume issue that I expect is the folk's next door ! the room itself is quite small too but this combo would not have any problems in a much larger room. I am basing that guess on the fact that I can hear it perfectly when I check the mail! I hope you get it sorted out because when you do,I am sure you will be impressed I am not just impressed ,I am honestly amazed, this amp for this price is a steal . I (like many others i'm guessing) have spent alot of money on other equipment and then been disappointed. I am definitely very happy and also with the 'less equipment' (no pre amp) I am using less total tubes and so have less of same to monitor and replace etc. So as for myself I am thrilled .Scott

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