K&K Audio / Lundahl Transformers

Welcome! Need support, you got it. Or share you ideas and experiences.

Return to K&K Audio / Lundahl Transformers

St-70 Premium Kit

129.242.34.99
Posted on April 22, 2009 at 04:18:10
Arober
Audiophile

Posts: 9
Location: Up north
Joined: April 22, 2009
 Lookin for an amp upgrade I have a few questions about this one.
- What is the input sensivity. Its important to me cause I have a delicate gain structure in my system.
- I do not have the original OPT but a pair of 5K/40W hasimotos. I guess stability is not an issue as there is no feedback, but are there other things that could be an issue regarding OPTs and this kit.
- I would like to mount the kit inside the chassis, turning it upside down and with wires to tubesocket on the top plate. Would it work and how high does the kit mount parts and mounting included?
- I have 4 quite costly 0,047 coupling caps in my LTP-driver right now. Could I use them in the kit. I guess it could mean an early roll-off but by how much?

RE: St-70 Premium Kit, posted on April 22, 2009 at 04:36:07
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 With the input transformer wired for 1:2, the overall gain is 16dB.

Other 4K-5K output transformers will work just fine.

If you truly mean turning the PC board upside down with the components pointing down, it will not work due to the height of the main heat sinks. You can place the board under a chassis top right-side up spaced down 0.75", mount most of the parts on the bottom of the board, and relocate the output tube current sources to a heat sink attached to the back of the chassis. This has been done successfully by at least one builder.

You can compensate for the lower value of the capacitor by increasing the value of the output stage grid leak resistor, so that you won't raise the roll-off frequency by enough to matter. In any case, using the stock resistors and 0.047uF capacitors, the -3dB point would be 25Hz.

Kevin Carter
K&K Audio
www.kandkaudio.com

RE: St-70 Premium Kit, posted on April 22, 2009 at 04:41:07
Arober
Audiophile

Posts: 9
Location: Up north
Joined: April 22, 2009
 Kevin: Forgot to mention that I have a homemade/ custom chassis, thats why I thought of going inside and then need the hight data.

RE: St-70 Premium Kit, posted on April 22, 2009 at 06:21:30
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 The tallest component on the PC board (other than the large heat sinks, which can't be located inside of a closed chassis due to heat), is the tallest filter cap, which is 1.8" tall. Adding that to the 0.75" that the PC board must be mounted below the top panel to clear the input transformer, which can't be mounted on the bottom, it appears that you will need roughly 3".

I wouldn't build a chassis without having the kit in hand to make sure you are doing the right thing.

Kevin Carter
K&K Audio
www.kandkaudio.com

RE: St-70 Premium Kit, posted on April 23, 2009 at 01:11:13
Arober
Audiophile

Posts: 9
Location: Up north
Joined: April 22, 2009
 Thanks for tips.
How do you express the kit input sensivity in voltage. It is something with logaritmic scale to do the conversion?

RE: St-70 Premium Kit, posted on April 23, 2009 at 04:06:10
AnandR
Audiophile

Posts: 688
Location: North Carolina
Joined: September 8, 2003
 I'm sure Kevin will be able to express the kit input sensitivity in voltage but the more important question is the overall gain structure of your entire system? What is the sensitivity of your speakers? What is the overall maximum gain of your preamp? What is your source? Vinyl or Ceedee?

Most systems have way too much gain. And most recordings drastically vary with how loud the recording is within the same album! So input sensitivity won't help you much, but knowing the overall gain of your system or at least the goals will.

Anand.

RE: St-70 Premium Kit, posted on April 23, 2009 at 04:14:26
Arober
Audiophile

Posts: 9
Location: Up north
Joined: April 22, 2009
 I know, AnandR. My hole system is diy and a bit complicated when it comes to gain structure with different sources and tvc volume controls. As I am used to express it in voltages it is this data I need to see how the kit will fit.

RE: St-70 Premium Kit, posted on April 23, 2009 at 08:31:37
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 The input sensitivity is about 0.9vRMS.

Kevin Carter
K&K Audio
www.kandkaudio.com

RE: St-70 Premium Kit, posted on September 24, 2009 at 01:42:39
Arober
Audiophile

Posts: 9
Location: Up north
Joined: April 22, 2009
 Having troble fitting this (st-70) kit into the chassis I have already buildt for another circuit I tried to do som reverse work on the ST-70 kit. It is not easy to see how the output tubes operate. In class A you need a high current, a lower Va and a lower bias voltage for the output tube. I guess you run about 50mA per tube in the kit, but if I build the amp somewhat differently how low can I go on the bias voltage before the CCS drops out of regulation and is there any "rule of thumb" on how high impedans the cathode should see down in the CCS?

RE: St-70 Premium Kit, posted on September 24, 2009 at 05:24:43
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 I'm not sure exactly what you are asking, but the output stage runs Class A differential, because both tubes are sitting on a common cathode current sink. In this case, it doesn't actually matter what the bias point is, as there is no possibility of crossing over to Class B operation, however, the bias point you use determines how much power you can get out of the amp. The CCS never drops out of regulation from zero signal to clipping. If you are thinking that you want to create a more "conventional" sort of Class A bias point with lower plate voltag, low grid bias voltage, and higher bias current, you don't have to worry about the CCS either. With reasonably well-matched tubes, you don't see more than a few hundred millivolts of AC from the common cathode point to ground at large AC signal levels. This is one of the consequences of using a CCS rather than a resistor in the common cathode position. The cathodes "see" megohms "looking" toward ground.

Kevin Carter
K&K Audio
www.kandkaudio.com
RE: St-70 Premium Kit, posted on September 24, 2009 at 06:48:19
Arober
Audiophile

Posts: 9
Location: Up north
Joined: April 22, 2009
 Thanks Kevin. This made me understand a lot more.
My concern is that if you bias up an output tube with a CCS "under" the cathode and the cathode then is lifted to a positive voltage while the grid is grounded, then there is a voltage potential across the CCS. The CCS needs this to work properly, and there is for a specific CCS a minimum voltage reqiurement for proper functionality. If you lower B+ and increase current thru the tube then this voltage drops toward zero, and the CCS might fall out of "regulation" - right? For a EL34 tube what is normally understood as "class A" operation is like 250Va/100mA, and the cathode voltage would be like up 12-13V relative to a grounded grid, and that is not sufficient for any kind of CCS.
I understand now better how the kit works, it was just the "class A" consept that confused me when I started thinking about how the tube output stage worked. "Forced class A" it is no matter how you bias the tube, as I understand now. Thanks again.



RE: St-70 Premium Kit, posted on September 24, 2009 at 11:50:34
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 The drop out voltage for the cascode CCS based on depletion mode MOSFETs is about 5v + 1/2 RMS voltage swing. Used the way they are in the ST-70, there is no voltage swing, so drop-out wouldn't occur until about 5vDC is reached.

Kevin Carter
K&K Audio
www.kandkaudio.com

Page processed in 0.088 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2009, Audio Asylum, all rights reserved ] [ Go Green ] [ Take5 ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]