OPA454 Front End for LuFo Amp

[xrk971]

This is a new Yarra/M2X format daughterboard that will be used to drive the new 39w LuFo SE Class A amp. I decided to use an OPA454 opamp capable of +/-50v rails and 50mA output. We will power it with a custom +/35v DCDC converter on the lower mezzanine and coupled to the upper mezzanine through a 3pin header. The DCDC converter will take +18v to +30v input from the main amp board. It uses LT8364 converter with SEPIC topology and a coupled inductor. JPS64 has done the beautiful layout and Jhofland designed the DCDC converter. I have designed the OPA454 to provide the LuFo with 22.4dB of gain. This should be perfect when driven by an external preamp with circa 6dB of gain. The OPA454 was chosen since it has capability to handle up to +/-50v rails and can drive up to 50mA of output and had only 0.0007% THD.

We have made provision for a nice Mundorf 2.2uF EVO M cap on the input. It will be DC coupled output and AC coupling will be handled on the main LuFo amp board.

 

[poseidonsvoice]

Wow. Super! And just about the right amount of gain for us fellas who don't have a preamp! Will it offer differential to SE conversion or should we use BTSB?

Best,
Anand.

 

[xrk971]

It is SE input. If you have balanced, you can use BTSB to convert to SE first. Use 6dB setting on BTSB coupled with 22.4dB gives about 28.4dB - a perfect amount of overall gain.

 

[xrk971]

Some renders of the Gerber files that I am using to order the verification build PCBs with.

 

[marconi118]

Will you post the schema of this board? More interested in the preamp than the PSU

 

[xrk971]

Sure, here you go. Nothing special - just a non inverting opamp with gain of 22.4dB and input RFI filter. +/-35v rails with a ferrite bead filter. DC coupled output.

 

[xrk971]

Here is the assembled OPA454 front end double stack. The lower board is the DC/DC +/-35v PSU and the upper board is the OPA454 opamp fed from the lower board through a 3-pin header. The signal in and out pass through the lower board to the upper board.

Lower board:

Double stack:

Side view:

End view:

Closeup showing the Susumu 0805 chip resistors and Vishay Beyschlag MELF resistors (wherever the audio path is):

 

[xrk971]

I am really impressed with this little preamp of front end module. It’s a small but mighty package. It sounds really good with a low noise natural presentation. 0mV DC offset at the output. Drives the LuFo and now tested on the 50w Glass Harmony by Hugh Dean. Being able to use 12v from a Wall wart or DC rail from main amp supply at 28v or whatever is awesome.

Here is is driving the 50w Glass Harmony:

 

[marconi118]

Where to find more info (schema) on Glass Harmony?

 

[xrk971]

Hi Marconi,
The Glass Harmony is a commercial design by Aspen Amplifier, P/L, owned by Hugh Dean, aka AKSA on DIYA. So it is not a public design.

It is perhaps one of the finest amplifiers I have heard anywhere.

 

[xrk971]

I forgot to update this thread. These boards are now used in the SuSyLu to produce 80Vpp swing in a balanced BTL amp.

 

[xrk971]

Manufacturing Data for OPA454 Front End

Here are the complete manufacturing data files for the HV opamp front end for the LuFo/SuSyLu amp.

I will also provide these through my shop for those who do not want to order their own boards. They will be 1.6mm thick, 1oz copper, ENIG finish, green solder mask. Sold as a panel consisting of a power supply sub-mezzanine and the opamp board on top. You will need two for a stereo LuFO and four of them for a stereo SuSyLu.

Note that the OPA454 is almost impossible to find at present. They are available from LCSC. Or you can use the OPA445 - just do not populate C124/R124 in order to leave pin 8 N/C.

A big thank you to JPS64 for making yet another super layout for this front end.

Btw, you can also use this in the Yarra for a very high swinging output preamp. Also fits in the M2X amp.

 

[Plott]

Hi X,
great! You wrote about the M2x... is there a possibility to use the M2x front ends too? SMD is not for everyone Or will you sell populated FE boards too?

 

[xrk971]

Yes, you can use Melbourne to drive LuFo. It will need an external +/-30v PSU. LuFo has a provision for a Molex KK 3-pin jack for external power for the FE.

 

[Plott]

Thanks X!

 

[xrk971]

I have found that the LTC6090IS8E#PBF is a drop in replacement for the OPA454 which is hard to find. The LTC6090 can also do 140vpp output. Just leave pin 8 not connected. So DNP the 30pF cap and 1MOhm resistor on pin 8. If you jumper pin 8 to pin 5 it provides auto thermal shutoff.

https://www.mouser.com/ProductDetail/Analog-Devices/LTC6090CS8EPBF?qs=hVkxg5c3xu9WAyVAY81tSw==

Here I am testing it with a square wave test 19.4Vpp output. I am feeding it 3.3v and gain setting is 15.8dB. PSU is a +/-30v DCDC using 12v in.

Anyhow, a 140Vpp opamp with 50mA drive capability really could be a nice front end and driver stage for a simple output stage. That’s good for a 300w into 8ohm amp.

 

[jan.didden]

Sure, here you go. Nothing special - just a non inverting opamp with gain of 22.4dB and input RFI filter. +/-35v rails with a ferrite bead filter. DC coupled output.

XRK, don't you want a small resistor in series with the opamp output to isolate cap load?

Jan

 

[xrk971]

Good point - we should have a set of pads to allow install of optional series resistor (maybe 600ohm?). This module was meant to be used with another board that should have its own series resistor. I still need to make an update on this board so this will be a good change to have. Thanks.