Need to build JC 2 preamp - Page 2

Elso Kwak · 21st January 2003, 09:49 AM

Quote:

Originally posted by halojoy
/halo - likes to increase the voltage in designs
- get more Tube-like

Hi halojoy,
You raise a interesting point regarding the difference between tube and sold state circuits. Mr. Metaxas has some interesting thoughts about this:
http://www.metaxas.com/pages/masnewfiles/index.html
(Click on technology, then on voltage regulation)

halojoy · 21st January 2003, 10:43 AM

Thanks, I will see what he thinks.

I know I have been on that website once before.
Probably some year ago.
I recognize some of the names of the amplifiers.

Thius is some of what he, Kostas Metaxas AUS, has to say:

Quote:

The most significant difference between VALVE and TRANSISTOR circuits is the amplifier/power supply regulator circuit interaction which is far more critical in Transistor circuits.

To illustrate this phenomenon using the the most basic amplifier gain stage, let us assume that our input signal is a 1.0 Volt peak to peak, 1 kHz sine wave and the amplifier stage has a gain of 10 so that the output voltage is 10 Volts peak to peak. For simplicity, resistor R is the load resistor which dictates the overall gain.
If this was a VALVE amplifier, the high voltages which are typical in valve circuits (from 200-400 Volts DC) would result in a valve of around 50,000 to 100,000 Ohms for resistor R.

The equivalent transistor amplifier using much lower voltages (from 12-15 Volts) would have a substantially lower value of R between 200 Ohms-100 Ohms. Therefore the power supply used in the transistor amplifier is virtually directly linked to the transistor amplifier circuit compared to the isolation of over 50,000 Ohms in the Valve circuit.

sonnya · 21st January 2003, 12:40 PM

Quote:

Originally posted by HKC
Hello Elso & Halojoy

Thank you for the information for JC2. The Sonny Andersons circuit also interested me. I will make a decision to choose one among the two circuits.
Thanks again for all the inform.

You could easily replace the J113 in the inputpair with 2sk389 if you want to.

For Halojoy :

I will make a version for you with 2sc2240 and 2sa970. i believe you will order BL parts? am i right?

Elso :

Should i make it a stereo board, smd or leaded resistors?

By the way this circuit could very very easy be enhanced to be an power amp.

Elso Kwak · 21st January 2003, 03:33 PM

Quote:

Originally posted by sonnya

You could easily replace the J113 in the inputpair with 2sk389 if you want to.

For Halojoy :

I will make a version for you with 2sc2240 and 2sa970. i believe you will order BL parts? am i right?

Elso :

Should i make it a stereo board, smd or leaded resistors?

By the way this circuit could very very easy be enhanced to be an power amp.

Hi Sonny,
I don't like SMD. I did some experiments to convert my line amp to a poweramp. It worked till white smoke rose....

Powersupplyissues are even more important so it seems...
But I can make PCB's myself. I am just interested to see the schematic. Maybe I can learn something or I did overlook a circuitdetail in mines.

HKC · 22nd January 2003, 02:15 AM

Hello Elso

I read your post in the Audioaslyum before I posted this thread regarding the JC2 preamp. I am not familiar with electronic theory I would like to learn more about this circuit:
1. Does the circuit posted on the marklev.com is a workable circuit that I can knock off without any problem since you said the circuit is a very crude & early one.
2. Where should I take the measurement to read the quiescent current in the input stage?
3. How much quiescent current should I get by adjusting the 500R pot in the iput stage?

To Sonnya

I downloaded the pcb-top file but there is no image in it. Please advice what should I do.

Thanks again for everything.

halojoy · 22nd January 2003, 05:13 AM

Quote:

For Halojoy :

I will make a version for you with 2sc2240 and 2sa970. i believe you will order BL parts? am i right?

www.elfa.se
Here is what Smallsignal Bipolar I can buy:
http://www.elfa.se/elfa-bin/lt.pl?so...214943&2009911
-------------------

Here is what Small MOS & JFET I can select:
http://www.elfa.se/elfa/produkter/se/20/2009926.htm
-------------------------------------------------------------------

Has anybody some experience about using
the MOSFETs in DIL: IRFD120/IRFD9120
I see Pass have tried them.
But they have 360-390pF - That is much for a 1.3Watt device

caplocks say (in Thread about the very good MJE15030/31 transistors)
that MJE15030 have only 50-100pF
and that is a 50Watt 8Ampere !!!! TO220 Device

40 parallelled of IRFD120 would make 52 Watts
and something like 14.000pF !!!!!!!!
-----------------------------------------------------
Bad examples? Well I have read some bad maths & limping thinking
by people, trying to force Devices they do not like,
to seem bad...

I can say that BD139 have bad data for input pair in a RIAA amp,
and that it has poor performance at 10 Watt operation.
Does this say anything about the quality of BD139
No, because that is not what it is suited for.
To be a Input pair in very small signal applications.
Or to operate at 10 Watts
------------------------------------------------

/halojoy - hopefully doing some better math, next time

sonnya · 22nd January 2003, 02:34 PM

Hi halojoy!

You should be able to use the circuit with 2sk170gr instead of J113 and 2sa972sc2240 instead of 2n5401 and 2n5551.

but add some gain. I would post a feedback network you can use, so you would be able to run it and still have Zero DC gain.

HKC :

The pcb.zip should contain tiff pictures and they should be okay... Try again.

greyhorse · 25th September 2004, 06:16 PM

I built this circuit using GR (Idss: 2.6-6.5) grade 2sk389/j109s and ZTX450/550s, but forgot about the need to change the drain resistors. Can someone please tell me what value drain resistors I should use instead of the original 100ohm ones? Thanks in advance.

As for Fred's suggestion to cascade the J-FETs, my layout is kind of small, so I'd really like to get this circuit working with just the resistor change.

greyhorse · 25th September 2004, 06:53 PM

The full schematic:
http://marklev.com/JC2/jc2schematics.jpg

I've also attached X.G.'s corrected schematic as the site that was hosting it seems to be down.

john curl · 25th September 2004, 07:23 PM

Folks, the JC-2 is not an EASY design to make. Many of the original parts are obsolete. The circuit topologies are still viable, and almost typical, today. Thirty years ago, this was an advanced design. Elso has not been productive in improving this design, and while I respect his opinion, I find errors in his estimate of the sound quality potential of this design. Done right, it can still keep up with most anything today. There are some oversights, however, that we made in 1973-74, that need to be fixed. We attempted an upgrade to existing units in an article a few years ago in 'Positive Feedback' written by Chuck Hollander. It might be useful to find this article on the internet.

22nd January 2003, 02:15 AM	#15
HKC diyAudio Member Join Date: Jan 2003 Location: Hong Kong	Need to build JC 2 preamp Hello Elso I read your post in the Audioaslyum before I posted this thread regarding the JC2 preamp. I am not familiar with electronic theory I would like to learn more about this circuit: 1. Does the circuit posted on the marklev.com is a workable circuit that I can knock off without any problem since you said the circuit is a very crude & early one. 2. Where should I take the measurement to read the quiescent current in the input stage? 3. How much quiescent current should I get by adjusting the 500R pot in the iput stage? To Sonnya I downloaded the pcb-top file but there is no image in it. Please advice what should I do. Thanks again for everything.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
What preamp should I build?	OzMikeH	Tubes / Valves	154	21st January 2009 11:07 AM
BDT Preamp build	jkeny	Tubes / Valves	13	5th November 2008 12:37 AM
Build Preamp	Jpoole908	Solid State	1	8th June 2008 12:47 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

22nd January 2003, 02:34 PM	#17
sonnya diyAudio Member Join Date: Oct 2001 Location: Denmark	Hi halojoy! You should be able to use the circuit with 2sk170gr instead of J113 and 2sa972sc2240 instead of 2n5401 and 2n5551. but add some gain. I would post a feedback network you can use, so you would be able to run it and still have Zero DC gain. HKC : The pcb.zip should contain tiff pictures and they should be okay... Try again.

25th September 2004, 06:16 PM	#18
greyhorse diyAudio Member Join Date: Feb 2002 Location: Ontario	I built this circuit using GR (Idss: 2.6-6.5) grade 2sk389/j109s and ZTX450/550s, but forgot about the need to change the drain resistors. Can someone please tell me what value drain resistors I should use instead of the original 100ohm ones? Thanks in advance. As for Fred's suggestion to cascade the J-FETs, my layout is kind of small, so I'd really like to get this circuit working with just the resistor change.

25th September 2004, 07:23 PM	#20
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Folks, the JC-2 is not an EASY design to make. Many of the original parts are obsolete. The circuit topologies are still viable, and almost typical, today. Thirty years ago, this was an advanced design. Elso has not been productive in improving this design, and while I respect his opinion, I find errors in his estimate of the sound quality potential of this design. Done right, it can still keep up with most anything today. There are some oversights, however, that we made in 1973-74, that need to be fixed. We attempted an upgrade to existing units in an article a few years ago in 'Positive Feedback' written by Chuck Hollander. It might be useful to find this article on the internet.