CATO

Class-A Tube Line Level Amplifier with Single-Ended MOSFET Output

PLUS
Our New Power Supply Module!
 

This vacuum tube linestage kit, designed by Erno Borbely, is now available from Welborne Labs. Its all class-A operation makes for a very sweet sounding amplifier and its single-ended low impedance mosfet output stage is good for all the bass slam you could ask for! 

The uniqueness of this circuit design allows the CATO to wear many hats.  In fact it can be configured as:

-  a single-ended linestage with adjustable gain

- a unity gain buffer (gain of 1)

- a high gain microphone preamp (+40dB)

- I/V converter for DAC outputs (balanced or single-ended). 

Just about any audio circuit you could conceive of can be achieved with this module.

Features:
It boasts many very useful features making it a very versatile amplifier, including: a differential tube input stage with adjustable gain and a class-A single-ended MOSFET output stage with a very low output impedance providing high current drive capabilities.  MOSFET devices possess electrical characteristics that are similar to vacuum tubes and thus enhance the sonics of the driver stage. BUT, the most interesting feature of our CATO design is that it uses the ECC86/6GM8 low voltage tube.  The 6GM8 falls into the car radio tube category from the 1950's and 60's with specifications rated for linear operation provided with anode voltages supplied by a car battery, i.e. 12 volts.  The 6GM8 can be operated at higher voltages and in fact sounds better at these elevated potentials and thus we operate CATO from a + 24V supply.  But don't let the 6GM8 tubes scare you, CATO can also use the ECC88/6DJ8, 6922 and others. You will be amazed at just how good these high voltage tubes sound ran at low voltages. 

-  We have added a servo circuit to null all DC offset voltage.
This new circuit eliminates all dc offset so no large coupling caps are required on the inputs or outputs.

- We have added a dip switch providing 3 selectable gain settings.
Choose your resistors and have three selectable gain levels. 

- We now offer a new affordable power supply. 
We have optimized the amplifier design to operate best using an ac filament voltage and this has allowed us to greatly simply the power supply requirements saving you money. This new supply features encapsulated Toroidal transformers, Schottky diodes and AC filaments all mounted on a small circuit board.  Of Course the filaments can be run with dc voltage too.

If you are looking for an upgrade to your CD player please see what we have to offer here.
 

We have designed CATO as a mono line level amplifier, i.e. two boards are required for stereo operation and four boards are required for balanced input/output stereo operation. CATO is packaged on a silkscreened 4-layer printed circuit board measuring a very petit 3"(76mm) by 3"(76(mm). This 4-layer board features a dual ground-plane design providing a very low impedance return path and superior shielding for added immunity from noise. Each unit also includes on-board voltage regulation circuitry. These regulators can be bypassed if using a power supply with a more elaborate regulation scheme. The kit includes very detailed assembly instructions and all components necessary for proper operation are individually packaged in plastic bags and marked with their reference designators.

Circuit Description:
CATO is a DC-coupled hybrid tube/MOSFET amplifier.  It uses a + 24Vdc anode supply and 6.3V for the heaters.  The 24Vdc supply is provided by onboard voltage regulators.  We recommend powering on the filament voltage prior to applying the 24Vdc voltage, or better yet, leave the filaments powered-on all of the time and just switch the dc voltage at power on (this is how our new power supply is configured).

Its input stage is a differential amplifier with a constant current diode supplying the cathode current.

The two anodes, which produce two out of phase signals, are converted to a single-ended signal, using a current mirror circuit composed of Q1, D2, R3 and R4.  Q2, a 2SJ79 P-channel MOSFET, is used in a common-source mode as a class-A single-ended SE output stage.  Q2's drain resistance has been replaced with a second constant-current source to increase its gain and linearity.  This current source is made up of Q3, a 2SK216 N-channel MOSFET.  U3 and associated components implement the dc nulling servo circuit.

This circuit's open loop gain of 53dB is relatively high compared to most tube amplifiers and with an open loop THD measurement of 0.15%, it's also very linear.  This THD is mostly all second order too!  However, due to the high open loop gain, you can't use CATO without feedback.  Resistors R8, R25, R26 and R9 apply the feedback along with capacitor C2 which applies phase compensation adjusted for best square wave response.

The feedback resistors R8, R25, R26 and R9 adjust the gain of the amplifier which can be set anywhere from unity-gain (gain of 1) up to 40dB.  The new dip switch SW1 allows you to easily select the perfect gain setting.  The amplifier is not unity-gain stable and therefore the R15/C3 network is provided to roll-off the open loop response of the amplifier allowing it to work with low gain settings.  So if you want to use the amp with 6dB of gain or less, you need to install R15 and C3.  More information on these resistor and capacitor selections are provided in the CATO assembly manual.

As previously mentioned, due to CATO's unique design,
it can be configured for many different applications.
 

Figure 1 depicts the amplifier connected as a non-inverting amp with a gain of 20dB.  The gain is calculated as 20 times the LOG of R8 + R9 divided by R9.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5
 

Figure 6

The assembly manual includes step-by-step instructions for assembling and testing each module along with connection diagrams for configuring these amps for the different applications listed above.  We will try to post more application info on our website soon.
 

Be sure to order two amps for single-ended stereo operation.  For balanced operation order four amps.  Our power supply can drive two CATO modules.  Or use one power supply for each module (the best) and go completely dual-mono.

Each module - includes Vishay-Dale 1% metal film resistors, Nichicon ultra-low impedance electrolytic capacitors, Wima polypropylene film and foil capacitors and a 4-layer silk-screened circuit board.  Tubes not included, order separately - see below.

Power Supply - Includes two encapsulated low-noise toroidal transformers, Schottky low-noise diodes and Nichicon ultra-low impedance filter caps.

Available in kit form or have us assemble and test them for you.
 

ECC86/6GM8 NOS Tubes

PLEASE NOTE:  At this time, these tubes are ONLY sold to individuals that have purchased a CATO kit.

We reserve the right to limit quantities on tube purchases, or in other words "don't be greedy".

I haven't had a chance to listen to all of these tubes yet so the prices don't necessarily reflect the sound quality of a particular brand.  Accept for the Russian tubes all were manufactured in either Germany or Holland.

All tubes are tested, matched using a calibrated tester and warranted for 6 months.

NOS U.S.A. Branded tubes like GE, Sylvania, RCA, Zenith, Phillips, etc.  We choose the brand and send you a matched pair.

VALVO and Siemens

Valvo or Siemens brand - made in Germany  One matched pair.

AMPEREX

Amperex brand - made in Holland  One matched pair.

EH6922- ELECTRO HARMONIX

Electro Harmonix - made in Russia  One matched pair.

EH6922 GOLD PIN- ELECTRO HARMONIX

Electro Harmonix with gold pins- made in Russia  One matched pair.

HOME

SITE INDEX

CONTACTS