Bench Phono

[Nick]

Normally you would see a low inductance like 33H in a TX that was gapped for DC, a TX for no DC (eg) in a parafeed config would normally have a higher inductance.

I think you ar egoing to have problems with trying to drive that TX with that valve.

[richardcooper2k]

thanks nick. i've learned a lot finding out what i can't do

what would the effect be ? loss of freq resp or other problems ?

i may just try the choke loading without an o/p trans

but will also have a look out for higher L transformers

i don't feel ready to try changing the o/p valve yet - that's for the future !

[Nick]

Yep, low freq response will be down, and the lower load on the triode will mean more distortion in the lower freq as well.

If you look (for example) at something like the LL1621/PP when not gapped for DC that has a inductance of 300H.

Though I think its safe to say that the high ra valves are not designed for choke loading, you should look at the high gm/low ra valves if you want a choke/interstage and gain.

[Will]

O.K with charged batteries I measured the Altecs out of the phono.

Pri. and Sec. 160H @ 120Hz.

[richardcooper2k]

where do folk buy llundahl iron from ? not that i'm seriously consioring buying any at this moment but i never been able to find any

how do i work out the maximum output impedance for a stage for a given transformer ?

cheers

[andrew Ivimey]

K & K Audio in the US.

Kevin Carter runs a really good service with its own BB and threads a million about the sorts of things we're interested in. He is happy to talk or email, sends things out by return and his proces are good.

[Nick]

I have used Kevin in the past, recommended, also closer to home (and avoiding duty) are http://www.jacmusic.com/ amd http://diyparadiso.com/

[Mike H]

Can't help thinking there must be an easier way of doing this

[richardcooper2k]

hi mike. the background is i've built this phono and line stages pre amp based on audio note kits and now i'm playing with it/experimenting to improve the sound/learn

i'm open to any suggestions ! there are probably much better things i could be trying than the ones i am currently thinking about

i'm not comfortable putting the full circuit here but you can see the basic one on this page http://www.audionotekits.com/M3PhonoStage.html

HT is from a valve reg. all phono circuit valves are 6072

next thing i was thinking of trying is to decouple the cascode and output stage HT with resistors and caps and add a series 150H hammond choke to the anode resistor on the output stage to see what effect they have (one at a time)

also thought i'd try the same on the line stage with the choke loading on the input valve instead. i thought it would be better to replace the current LC decoupling with RC if i'm trying choke loading as two chokes may interact. i was also thinking the coupling between the two stages in the line stage might be a better candidate for an altec 15335a type parafeed bridging tranformer as Ra for 5687 is only 2-3k (just to see what difference it makes). line circuit here http://www.audionotekits.com/l3linemk2.html. power supply is a duplicate of the phono one

[Mike H]

Oh I see, so you're adapting from AN's line amp cct having the o/p TX for use in the phono amp?

And you're trying a choke load for V1 as well. Unusual, because chokes/transformers in a pre-amp are often bad news due to magnetic pick-up. See why you need large values though, e.g. 300H for V1 makes 100 kilohms @ about 50 Hz

So the object is so as not to use resistors I presume?

[richardcooper2k]

not quite, i wanted to try an opt on the phono as well as rather than instead of the line stage as i'm using both. but it seems i'd either need an expensive opt or to change the output valve so thats on hold for the minute

the chokes won't replace the load resistors as they don't have enough dcr to keep the same operating point. i beleive they will be closer to a constant current source than a plain resistor, which i beleive is a good thing

as to wether they'll cause problems or do something i like, i guess i'll find out if i try it

[richardcooper2k]

talking about vol pot position before or after line stages reminded me of a conversation i had with will at owston. he said he got a better sound putting his afterwards which now makes even more sense to me

but presumably putting one afterwards would affect the output impedance. now mine have a 5687 cathode follower with a 33:1 output transformer on the primary. so Ra is 2-3k but the anode load is the primary of the output trans so how do i work out the output impedance ? will it vary with frequency or is it the primarys dcr ?

the alternative i suppose is to put the pots in the power amp after the interconnect has been driven. but as mine are monos that becomes a bit of a pita. which if i remember rightly was one reason given by nick for going for buffer and tva

[Mike H]

Oh-kay, how about an actual constant current load using an extra valve?


Your transformer's impedance ratio is its turns ratio times itself. So if the turns ratio is 33:1, then the impedance ratio is 1089:1.

That means the primary impedance is whatever is the impedance of the thing that is connected to the output secondary, x 1089.

If you want the transformer to present an anode load of 3k then the thing connected across the secondary has to be 3k/1089, or 2.7 Ohms.

[richardcooper2k]

hoorah ! i've scored 2 NOS altec 15335A on ebay for £23 - bargain.....not quite sure what i'm going to end up doing with them yet though.....

constant current load - not sure how to implement these or what the benefit would be - i'll have to do some more reading

impedance - i follow the ratio bit but i'm still not clear how to work out what the output impedance of my line stage will be as it's got the primary of the opt as the anode load rather than a resistor

[Mike H]

I've been trying to find this again and finally found it ~ example of a constant current source (V2):

http://www.izzy-wizzy.com/audio/vw_precct.html