Projects' Details

How it is built

My inspiration: Housing was Chord SPM 14000 Ultimate Mono Power Amplifier

My realization: Draft of my Alu housing


English basic source of Krell KSA 50 I found in diyAudio

Polish basic source of Krell KSA 50 I found in audiostereo

Some photos of Krell KSA 50 to investigate the device inside

Naked Krell KSA 100 to deeply investigation

Image 1: Original Schematic of Krell KSA 50Mk2

Image 2: Original PSU Schematic of Krell KSA 50Mk2

Image 3: Original Twicked PCB of Krell KSA 50Mk2

Image 4: Schematic of inputs and drivers

Image 5: Schematic of outputs transistors

Image 6: BOM of my amplifier

Image 7: PCB of my amplifier

Image 8: Schematic of my PSU

Image 9: BOM of my PSU

Image 10: Schematic of my Soft-Start

Image 11: BOM of my Soft-Start

Image 12: PCB of my Soft-Start

Image 13: Schematic of my Speaker Protector

Image 14: BOM of my Speaker Protector

Image 15: PCB of my Speaker Protector

Image 16: Schematic of my Input Sockets

Image 17: BOM of my Input Sockets

Image 18: PCB of my Input Sockets

Image 19: Schematic of my Balanced Input Driver

Image 20: BOM of my Balanced Input Driver

Image 21: PCB of my Balanced Input Driver

Image 22: Schematic of my Fans Driver & Heat Controller

Image 23: Schematic of my Sub-Power Supply

Image 24: BOM of my Sub-Power Supply

Image 25: Schematic of my Balanced Input Driver PSU

Image 26: BOM of my Balanced Input Driver PSU

--------------------------------------- Datasheets of parts:

Datasheet 1: 1N914

Datasheet 2: 1N4001/4007

Datasheet 3: 1N5333B/5388B

Datasheet 4: 2SA968

Datasheet 5: 2SA992

Datasheet 6: 2SC1845

Datasheet 7: 2SC2238

Datasheet 8: PEH 200

Datasheet 9: ZL

Datasheet 9: CD10-42 CDV19-30

Datasheet 10: FKP2

Datasheet 11: PHE 450

Datasheet 12: NF2D

Datasheet 13: NAC 3MPA

Datasheet 14: NC3FD-H

Datasheet 15: MFB40H12

Datasheet 16: MFB60D05/60D24

Datasheet 17: SK68-75-SA

Datasheet 18: SK125-84-SA

Datasheet 19: INA134

Datasheet 20: TLH42XX

Datasheet 21: MJ15003/15004

Datasheet 22: MJ21193/21194

Datasheet 23: MJE15030/15031

Datasheet 24: 3296W

Datasheet 25: 45 Serie

Datasheet 26: DS Serie

Datasheet 27: G6L

Datasheet 28: LBA110

Datasheet 29: RT2

Datasheet 30: CMF (RN, RL)

Datasheet 31: MRA

Datasheet 32: uPC1237

Datasheet 33: VBE 55-06 NO7

DualMono.Class A

...My DIY audio world...
Krell KSA-50 Inside


Why the DualMono Class A® ? Because is GREAT. No, is GORGEOUS. No, is MAGNIFICENT. No, is SPLENDID. No, no, no, all words together are too less to describe HIM. He is His Majesty KRELL KSA 50. Hough!

How it was made

The DualMono Class A® is designed as dual mono power amplifier with dual mono symetric power supply. It is equipped with soft-start circuit, dual automatic driver of fans, dual output protector, balanced inputs driver and dual small power supplies dedicated for them. All will be enclosed in solid screwed Alu housing with big heat-sinks ventilated by four fans. Design of housing was inspired by other great and sophisticated amplifier: + CHORD SPM 14000 ULTIMATE MONO POWER AMPLIFIER +. I hope you have the same opinion as me: this is fantastic design and very unconventional or rather unorthodox. The DualMono Class A® will get nearly copy of the CHORD's housing but with different internal heat-sinks. My device will have the same top plate with big holes covered by metal network but through holes you will can see two big heat-sinks along both sides, big bank of four big electrolytic capacitors. My housing will have similar six Alu "columns" on both sides and four solid Alu feet. On the back panel will be two heat-sinks work as cover for four fans. Between them will be 10mm thick Alu plate with sockets: inputs on BNC, RCA, XLR with balanced/unblanced toggle switch and loudspeakers plugs and 230V PowerCon socket. So the housing will be very heavy and with internal parts the DualMono Class A® will be extremely heavy. I estimated that total weight will be around 60 lbs. I wonder in which way I will can transport the device from my workshop (by car) to my house. Maybe I will need any workers to support me. I hope I will do it.

Many years ago I start to read post about clone of Krell KSA 50 on diyAudio and later on Polish Audiostereo forum still think about the project. There I found original schematic of the Krell KSA 50 and in last year I decided to built electrical copy of the amplifier with copy of the original pcb opposite to projects published by DIY'ers on both forums. I want to clone of the amplifier not different amplifier built on different pcb with different transistors and other components. I have designed copy of the original pcb and have bought original Toshiba and NEC transistors. Rest the same components (power transistors, resistors and capacitors) I have bought at Mouser Electronics. My idea was to build a clone. It is general base of the DualMono Class A®. Below are details about each circuit/module.

The DualMono Class A® circuit is built on 2SA992/2SC1845 NEC, 2SA968/2SC2238B Toshiba and MJE15030/MJE15031 ON Semi transistors and as power drivers work MJ15003/MJ15004 ON Semi or MJ21193/MJ21194 ON Semi. I do not know what type of these power transistors I will choose yet. They give different sound so probably they need to make some
3500 PCB Neked Overview
experiments. MJ15003/MJ15004 give soft treble rather but MJ21193/MJ21194 give more detailed (or sharper) treble. The pcb is made of 3,2mm thick double layer FR-4 laminate with 100um Cu. Dimensions of it are 140mm x 153mm. The pcb is very stiff and rigid. On the pcb are two heat-sinks made by Fischer (type: SK125-84-SA and SK68-75-SA): first for two pairs 2SA992/2SC1845 and two pairs 2SA968/2SC2238B and second a bit bigger heat-sink for MJE15030/MJE15031. Power transistors will be screwed directly to main ventilated heat-sinks (with ceramic isolators, of course). I use the same type of resistors like Krell used: CMF 65 Vishay Dale and 3296W Bourns potentiometer, except emiter resitors of drivers MJE15030/MJE15031. There are MRB-5Ni type made by Mills. And emiter resitors of power transistors MJ15003/MJ15004 are MP930 made by Caddock. Caddock's resistors are screwed to main heat-sinks near power transistors. Electrolytic capacitors are ZL type made by Rubycon and film capacitors are FKP2 made by WIMA and PHE450 made by Evox Rifa. Tere are two Silver Mica capacitors of CD15E type made by Cornell Dubilier.

Power Supply is the most important modul of the DualMono Class A®. There are two
3500 PSU Toroids Modul
big to audio dedicated toroids 450VA each. Each of them gives 2 x 26,5V with 2 x 8,7A. Both toroids are screwed together and fitted into 5mm thick 160mm diameter round steel sub-housing works as a shield. The shield has all walls damped by 2mm thick cork plate. Wires go out through holes drilled in places near toroids connections. The module will be screwed to 15mm thick plate made of cotton textile-phenolic resin. The plate will be screwed with cork palte directly to bottom wall of the amplifier housing. Such complicated construction should remove the most resonances come from toroids.

Second module of power supply is bank of electrolytic capacitors. I use
3500 PSU Caps Modul Opened
one big 100000uF/63V capacitor of type PEH 200 made by Evox Rifa in each rail so the bank consists of four capacitors. They are screwed to 10mm thick teflon plate with 5mm thick washers (each washer has the same diameter as capacitor). And next each capacitor is screwed by big nylon cap made by Evox Rifa too. This modul is screwed to 15mm thick plate made of cotton textile-phenolic resin with four holes for nylon caps and one hole in center with small 40mm diameter fan. The fan is use to cooling capacitors module. To the resin plate are screwed four Alu walls so it create second sub-housing. On top of the sub-housing will be
3500 PSU Caps Modul Bottom
screwed two FRED rectifier bridges type VBE 55-06 NO7 (600V 68A 35ns) made by IXYS. They will be fitted to flat heat-sinks. Ground rails between capacitors are "connected-bridged" by 5x15mm thick copper rods. Each rod is screwed to capacitor's clamps by M5 screws. The capacitors module will be screwed with 2mm thick sheet of cork to the bottom wall of the DualMono Class A® housing. I hope capacitors and rectifier will be enought isolated from any resonance come from toroids. Based on other DIY'ers experience Evox Rifa electrolytic capacitors should be good choice for good sound and deep and powerfull bass. The Fast Recovery Epitaxial Diodes (FRED) should be good too but it is my choice only. How it will play? I do not know at this moment but it will be any trouble to change them to other type of rectifier bridge.

Using such big toroids would be dangerous to electricity network of my house so DualMono Class A® will be equipped with 2000VA Soft-Start circuit. The circuit I "dragged" from Sjostrom Audio named as Sjöström Mains Circuits: SST-01 The softstart for toroid transformers and I modified it a bit. I designed
3500 PCB Soft-Start Naked Overview
dual sets of relays: one for left channel toroid and second for right channel toroid. And I added input for thermistors of both channels which drive the same relays as Soft-Start to switch off mains input (brake) from the DualMono Class A®. Used relays RT424024 type made by Tyco Electronics/Schrack are high current type (8A) so the Soft-Start can drive nearly 2000VA toroid in each channel. Input pins for two thermistors (serial connected for both channel) will be break ("open") in case when one of these thermistor will be overheated. Thermistors are screwed left and right main heat-sinks. The Soft-Start circuit had a LED driver for display Power On/Off. The pcb is made from the same FR-4 laminate like amplifier: 3,2mm thick with 100um double layer copper. Rigid pcb for high currents.

To avoid any damage of my speakers I have designed dedicated a Speaker Protector for my DualMono Class A®. The circuit is based on IC uPC1237 type made by NEC. Each channel has independent circuit so the Speaker Protector is in dual mono configuration. The uPC1237 is integrated circuit needs few passive components. Values of each component I calculated according to datasheet. But maybe some of components will need any recalculation during initial work. The circuit feature following functions: Output Offset DC Detection, AC Level Detection, Overload Detection, Automatic/Latching Resetting, Time Delay from Power Amplifier Power Switch On to Relay On, Wide Range of Power Supply Voltage (25 - 60V). In spite of patent protection the Overload Detection is not apply in the IC, so I use only socket to connect to it external circuit of Overload Protector (maybe the same like I found in schematic of Rotel RB-1090 amplifier. A project of pcb apply LED soldered directly in the pcb as well via connector to LED fitted in front plate of DualMono Class A®. At this moment the pcb is designed and Gerber files wait to send to workshop.

Module of inputs consists of two circuits on two pcb. The DualMono Class A® has switched balanced and unbalanced inputs. Balanced inputs use Audio Differential Line Receiver INA134 type made by Burr-Brown in dual mono configuration, of course. XLR sockets are connected to differential inputs of the IC and RCA/BNC sockets are connected directly to the toggle switch. The switch connects outputs of the circuit to INA134's outputs in balanced mode or connects to RCA/BNC sockets in unbalanced mode.

The DualMono Class A® works in Class A so two 50 Watts channels generated a huge of heat. Both main heat-sinks are ventilated by four fans and have overheat protectors. I designed a special circuit to drive all fans depending on temperature of heat-sinks. The circuit drive a small 40mm dia fan fitted under bank of power electrolytic capacitors too. More details about the circuit I will publish after the pcb design will be finished.

The last module of the DualMono Class A® is sub-power supply consist of few sub-modules: dual Speaker Protector PSU, dual Fans Driver and Heat Controller PSU, Caps Fan Driver PSU, EMI/RFI Filter of Balanced Input Driver and dual Balanced Input Driver (Regulator) PSU. More details will be published after all pcb for above circuits will be designed.