DIY Audio Projects Forum

KT120 Oddblocks
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Author:  laurie54 [ 06 Jun 2017, 19:58 ]
Post subject:  Re: KT120 Oddblocks

I'll add to what Bruce has said.
Also the optical light pots are pron to changing value over time which means that one channel will eventually start being louder or softer than the other. Another problem is the photo cells do not all have the same change in resistance as the light gets brighter resulting in the same problem, mismatched channel levels. All this means buying many of the cells and then testing each to get the closest to each other in pares.
Much better off to buy a rotary volume control. (stepped increments), they do not suffer from getting dirty as fast as most pots do. !% tolerance and better resistors are very cheep so the cost is in the rotary switch which where it should go.
I have used digital volume controls. Most are excellent,in dynamic range, tracking, and have a range of steps which is more refined than most ears can hear so 1, 2, or 3 db per step is common, but, the better ones are not cheep either and many require intelligent control so a CPU or PIC and program is needed to run them. It is possible to use strait digital logic for controlling some of them but even that requires some type of pcb or vero board, power supply, etc to get going.
Really really really hard to tell a good pot from digital IC / optical / or rotary. Get something that fits (size wise), is good quality (doesn't change value or mis-track), that feels good to the hand when rotating it, and doesn't get so noisy that changing it becomes a must for ones sanity. I guess it depends on whether you want something you can install and enjoy or something you can experiment and play with to tailor to your own needs.

Author:  laurie54 [ 06 Jun 2017, 20:29 ]
Post subject:  Re: KT120 Oddblocks

Here is just one of many companies selling the rotary types.

You will find the Alps "Blue Velvet" pots are popular in audio.

Luck to you.

Author:  Geek [ 07 Jun 2017, 00:30 ]
Post subject:  Re: KT120 Oddblocks

laurie54 wrote:
Here is just one of many companies selling the rotary types.

I've used those. I can vouch for them :D

You will find the Alps "Blue Velvet" pots are popular in audio.

Watch out for fakes!
Stick to reputable dealers... Partsconnexion and the like. Give most Ebay and AliExpress shops a pass :censored:


Author:  ozapaydin [ 07 Jun 2017, 02:27 ]
Post subject:  Re: KT120 Oddblocks

Thank you all for recommendations.

Author:  ozapaydin [ 08 Jun 2017, 02:47 ]
Post subject:  Re: KT120 Oddblocks

Hi Geek,
Well, due to convincing negative feedbacks againts lightspeed attenuator, I'm returning to a normal passive amp design.

There is no earth grounding in the circuit you posted, only chassis grounding if the case is metal, right?

It looks like a shielded cable is used inside the passive amp. So, to complete grounding against RF and EMI, we should use both side terminated shielded phono cable to give unwanted RF and EMI to the source side (CD player,etc.). If we use such a cable, doesn't it create another ground loop while avoiding one?

(Both side terminated shielded phono cable: Shielding is soldered to the connector on both sides of the phono connectors.)

If we use a metal case, shouldn't we earth-ground it?

Geek wrote:
ozapaydin wrote:
I also plan to make a passive preamp for kt120 oddblocks. What kind of case should I prefer? Metal or wooden?
If I use metal case, how should be the case shielding and grounding?
If I use wooden case, case will not protect from EMI? Does it introduce some noise into the signal?

Here's a passive pre circuit for lowest noise and no ground loops...

Bruce, got another? :idea:

Author:  poty [ 08 Jun 2017, 06:44 ]
Post subject:  Re: KT120 Oddblocks

ozapaydin wrote:
There is no earth grounding in the circuit you posted, only chassis grounding if the case is metal, right?
Not exactly. You've correctly noted that
ozapaydin wrote:
If we use a metal case, shouldn't we earth-ground it?
the case is not grounded in any point. So
ozapaydin wrote:
to complete grounding against RF and EMI, we should use both side terminated shielded phono cable to give unwanted RF and EMI to the source side.
is correct. (I assume that "shielded phono cable" is one wire in the shield.) In the case you always have to "terminate" the cable to both sides or the return path would be broken. There is no ground loop in this case (beside the ground loop between channels).
If you think about "2 wires in the shield" cable or "double shielded" cable (one wire and two shields) then one of the shield (external) may not be connected from one end and that way breaks the single channel ground loop.
Personally I would not ground the case and more than that - not used any connection from a signal ground to the case in such passive design.

Author:  Juancho [ 08 Jun 2017, 07:01 ]
Post subject:  Re: KT120 Oddblocks

I would not recommend a passive pre to go with the KT120 Oddblocks unless you have a high output input. These amps need a lot of drive. Whilst specced at 2v for max output I believe they need more drive than my Mastersounds also nominally 2v and I have my Forewatt's cranked quite high to get the same output.

Others experience may differ, but that's my few pennies' worth!

Author:  Lee De Forest [ 04 Sep 2017, 15:57 ]
Post subject:  Re: KT120 Oddblocks

I completed the construction of a pair of KT120 monoblocks in June, taking about four months to complete. This included the first 4-6 weeks shopping, ordering and waiting for parts. The amps use the originally-specified 12SL7 driver tubes. For inter-stage coupling, I went with Mundorf oil/silver 0.33 uf caps, bypassed with 0.022 uFJupiter Cu foil/waxpaper caps. The B+ supply uses only MKP (metalized polypropylene) film caps from Panasonic and Vishay - 5 stages in all. I find that film caps have much lower ESR and sound faster and more detailed, than electrolytics. The MKPs were bypassed with polystyrene film/tin foil types sourced from a Conrad-Johnson amp rebuild. In place of the 50R resistor, I took a pair of 60 ohm, 300 mA, 5 H chokes from Edcor. Chokes are way better at filtering out high-frequency hash that gets through the AC mains, than capacitors. Despite the higher DC resistance of the chokes, I needed to add additional resistance to bring the output tube B+ voltage down to the specified 450. For power supply and signal resistors, I used mostly non-inductive wirewound types from Mills and Ohmite audio gold. Values greater than 20k were audio grade PRPs. I took Mr. Heran's advice and doubled up on the LM317 HV regulators and took a heat sink rated at 3.6 degrees-C/W. The amps are built as a mirror-image pair with distinct left and right blocks. This is because I took pains to keep the wiring as short as possible; thus the speaker binding posts went on the sides, directly beneath the output transformers. For flexibility, I uses a pair of 100k Black Beauty potentiometers and installed in the back next to the RCA input jacks. It's nice to be able to turn down the inputs, when switching inputs or cables, or making some other upstream changes without having having to turn off the amp. Also, I could listen to the monoblocks without a preamp.

I listened at first with Svetlana 6550c tubes that I had on hand. It sounded terrific, although the tubes in one amp could not be brought completely into bias. Later one of them started to glow cherry red. It was then that I remembered that one of them was problematic when yanking out of my Conrad-Johnson Premier 11a. (This same amp now uses KT120 tubes). So I borrowed the KT120 tubes from the C-J, later replacing them. I listened to the K120 monoblocks at low (92 mA) and high (137 mA) bias current settings, and with/without a preamp (Heran's Forewatt with NOS Sylvania 12AU7 tubes, dropped into a Dynaco PAS 2 chassis, with greatly improved power supply over the original).

Except for the loss in output power there was no performance degradation in going without a preamp. (In solid-state amps in particular, I have noticed a loss in dynamic drive and weak bass when using a passive preamp). I drove monoblocks directly from a 2 gain-stage phono preamp, designed by Frank Van Alstine for the Dynaco rebuild kit that I couldn't help modifying with better parts, power supply and various tweaks to lower the noise. As the lack of output power when driving my VMPS RM-30 speakers (heavily modded, of course) was problematic with the occasional quietly-recorded LP, I usually keep the Forewatt line stage in place.

I don't hear a big difference in switching between low and high bias current with the KT120 tubes in place. Okay, with more current, there is more power, and the bass sounds slightly more controlled, but that's about it. As I have ample power to pressurize my 11x14' and arched ceiling listening room with low bias current and use of a line stage, I'm inclined to mostly go this route, especially while the weather is still warm. Likely, tube life will be extended. Using the KT120 tube in the C-J amp, built for KT88/6550 tubes, they seem to last forever, with the bias current barely shifting after 3 years. My overall impression of the sonics of the KT120 monoblocks is that they are very quiet, have lightning fast transients, impressive bass and extended highs, and excellent imaging. In short, they get everything right that I can think of, and are easily the best sounding amps I have heard in my system; and I've been through quite a few. Thanks Bruce for sharing your excellent design pour tout le monde.

Author:  Lee De Forest [ 04 Sep 2017, 16:06 ]
Post subject:  Re: KT120 Oddblocks

Greetings, I want to share a few photos of the monoblocks.

Author:  Lee De Forest [ 04 Sep 2017, 16:42 ]
Post subject:  Re: KT120 Oddblocks

Check out the photos.
Thanks for looking, Lee

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