Mauro's "Myreference Rev.C" building queries and guide.

[Hiten]

Hi Everyone,
Since Linuxguru's Group buy will take time I have started this thread for queries and guide. You can post design and amp building queries and share design idea's here. my queries are...
~ Is layout of above casing and pcb arrangement OK ?
~ Can we use Computer CPU Heatsink ?
~ What value volume pot is needed ?
~ Can we move volume pot near volume knob behind front panel ?
~ Does LM3886 Chip require any insulation while fixing it on heatsink ?
~ Will this amp require earth wire from AC power in ?
~ Which wires will be good for Line in and line out ?
~ Can we ground the metal casing so there is less interference ?
Everyone drawing is just rough layout please refer pcb for connection.

 

[linuxguru]

.
~ Is layout of above casing and pcb arrangement OK ?
~ Can we use Computer CPU Heatsink ?
~ What value volume pot is needed ?
~ Can we move volume pot near volume knob behind front panel ?
~ Does LM3886 Chip require any insulation while fixing it on heatsink ?
~ Will this amp require earth wire from AC power in ?
~ Which wires will be good for Line in and line out ?
~ Can we ground the metal casing so there is less interference ?

There are few errors in the connection diagram.

1) What has been labeled as LINE-OUT is actually SPEAKER-OUT, i.e. power output.
2) The connections from the transformer go to the connectors on the PCB which are labeled as AC1, PGND and AC2 (24-0-24) respectively, and not as shown.

You can use PC processor heatsinks if you can drill and mount it accordingly.
IIRC, the suggested value of the volume pot was 25k, but anything between 10k and 50k will probably work. Check the DIYaudio thread for details.
The volume pot can be on the front panel (that's where it usually is), but you'll need to shield the line-in cables carefully to avoid pick-up and hum.
The LM3886TF chip with isolated plastic package does not need insulation. The LM3886T with metal-tab needs to be insulated with a mica insulator or similar. I will be supplying the TF version only with the kit - it's safer.
The earth wire from the AC power line needs to be connected to the chassis ground, also known as the safety ground. The safety ground can be (optionally) connected to the amplifier power ground (PGND) through a CL60 isolator, or the PGND can simply be left floating.
For LINE-IN, you can use a shielded single-core cable for each channel.
For LINE-OUT (actually, SPEAKER-OUT), you can use standard speaker wire (this is usually transparent plastic insulated wire, with one core being bare copper (red) and the other core being tinned copper (gray), to indicate polarity. The tinned side goes to speaker ground (-), while the copper side goes to speaker (+).
Yes, you need to ground the metal chassis for safety - this will avoid an electric shock if by chance the live primary-side winding of the transformer accidentally shorts to the metal core and hence to the chassis.

 

[sachin_k5]

LinuxGuru,

Can you make the necessary corrections to Hiten's diagram and repost the picture ?

Hiten, Hope you don't mind.

 

[Hiten]

Thanks a lot linuxguru. I have changed original layout Pic. in first post. Few more questions. Please reply at your own convenience.
~ Can one single heatsink be used for both pcbs ?
~ I don't remember but I read somewhere if one includes earth wire to chassis ground it will create minute hum. is that true ?
~ If yes, can we isolate transformer from chassis and avoid earth wire from power line ?
~ Does pcbs needs to be shielded from transformer ?
~ Are LEDs on pcb necessary ?
~ How are Oground, Pground, Safety ground to be connected ?
Everyone drawing is just rough layout please refer pcb for connection.

 

[linuxguru]

.
~ Can one single heatsink be used for both pcbs ?
~ I don't remember but I read somewhere if one includes earth wire to chassis ground it will create minute hum. is that true ?
~ If yes, can we isolate transformer from chassis and avoid earth wire from power line ?
~ Does pcbs needs to be shielded from transformer ?
~ Are LEDs on pcb necessary ?
~ How are Oground, Pground, Safety ground to be connected ?

- Yes, a single heatsink can be used for both PCBs if the dimensions are sufficient.

- Hum is usually caused by 50Hz induction in ground loops - this is a complicated issue, but the simple way to avoid ground loops is to never connect a ground at two places. It should preferably be a single star-ground.

- Use the earth-wire from the power line for the chassis (safety) ground - it is a mandatory safety regulation in most places in the world. The only exception is if a double-insulated power-transformer is used - which may not be available locally.

- The grounding and track layout on the PCB is such that it needs no additional shielding. There is already a small-signal ground-plane on the PCB to provide shielding to the small-signal section.

- LEDs are optional. They only give an indication that the protection relays are active/inactive.

- OGND is the speaker return (-) point. PGND is the Power GND, which is connected to the centre-point of the power-transformer secondary (0 V). Safety GND on the PCB is optional - if connected, it is to be connected to the Chassis ground star point, and there must be a CL60 isolator installed on the PCB between PGND and Safety GND.

The modified connection diagram as seen now in the original post is conceptually correct. It is a bit simplified, in that there are 2-wire and 3-wire connections that are shown as a single line, but it's only a matter of detail.

 

[Hiten]

The modified connection diagram as seen now in the original post is conceptually correct. It is a bit simplified, in that there are 2-wire and 3-wire connections that are shown as a single line, but it's only a matter of detail.

Thanks again Linuxguru.
Yes diagram is simplified and is made to get basic idea. For accurate connection of all parts/components will PM you soon and post it here, as this will be helpful to new DIYers like me.
Regards

 

[keith_correa]

Thanks again Linuxguru.
Yes diagram is simplified and is made to get basic idea. For accurate connection of all parts/components will PM you soon, as this will be helpful to new DIYers like me.
Regards

Hiten, after collating answers to all your PM's, would appreciate you posting the questions & answers here. Would benefit many. Most of us are just getting our feet wet too.

Edit: Oh, sorry! I see that you already intended doing this.

 

[Hiten]

Yes will do that Keith. Will gather all remaining queries, Precautions to be taken and connection to be made and ask linuxguru in one go, as I don't want to bother him too much.
Regards

 

[quad]

Orienting the heatsinks to the rear or the sides, ensures
good air supply to cool it. Some examples (via google images) -

My_Ref Rev. C di Mauro Penasa

To keep signal lines short, one idea is to mount the volume potentiometer
on a L-bracket close to the RCA inputs and use a shaft extender
to control it via a front panel knob.

For the extension shaft, 6mm (1/4") rods are easily available in
aluminium, HDPE or brass. Both Aluminium and HDPE rods were about
Rs 10 per foot.

For coupling the extension shaft to the potentiometer, I used
the brass insert from a 60Amp (36 sq mm wire) terminal block, cheap and
easily available at electrical stores, esp the ones which supply industrial
lighting like Sodium Vapour lamps etc.

It looks like this -

 

[Hiten]

Thanks a lot quad. This is what I was looking for. 3rd pic of amp is yours, right ? Your layout will be very helpful to everyone. I have put vol. pot on back panel and all connection layouts are made perpendicular to each other. Does the heatsink gets very hot ?
Regards

 

[quad]

Yes, the 3rd pic is indeed my amp.
I have used the amp for multiple hours at a time without issue.
The heatsinks get quite warm, but not hot.

If I make another one, I would probably build it similar to this one -

 

[captrajesh]

Yes, the 3rd pic is indeed my amp.
I have used the amp for multiple hours at a time without issue.
The heatsinks get quite warm, but not hot.

If I make another one, I would probably build it similar to this one -

But my point is, considering that the Mauro kit is for Class D/T amp which has about 90% efficiency, is such a huge heatsink required?

 

[keith_correa]

There are these large caps at the signal input's. Anyone know what they are for?

 

[keith_correa]

But my point is, considering that the Mauro kit is for Class D/T amp which has about 90% efficiency, is such a huge heatsink required?

I'm sure the large heatsink is overkill but it sure does look noice :licklips:
Love the industrial looking speaker posts too. Haven't seen anything like them before. But then I haven't seen much

 

[captrajesh]

There are these large caps at the signal input's. Anyone know what they are for?

Keith, they are not at the signal inputs.

The power from secondary of the transformer is A/C current. The bridge rectifier converts it to DC.

If you observe closely, they are coming after the bridge rectifier (The one that converts AC to DC) and act as current bank (store current) to supply rush current for transient loads i.e. the snapping of kick drum etc.

 

[keith_correa]

Keith, they are not at the signal inputs.

The power from secondary of the transformer is A/C current. The bridge rectifier converts it to DC.

If you observe closely, they are coming after the bridge rectifier (The one that converts AC to DC) and act as current bank (store current) to supply rush current for transient loads i.e. the snapping of kick drum etc.

Nah, I'm not talking about those. What do you see connected to the hot end of the RCA jacks inside the enclosure?

 

[linuxguru]

But my point is, considering that the Mauro kit is for Class D/T amp which has about 90% efficiency, is such a huge heatsink required?

The LM3886 output stage is a conventional Class-AB amp, however, it's configured as a Howland Current Pump in the MyRef. Efficiency is a bit lower than the theoretical 78% max., but still in the region of 60-70% max. It doesn't get hot at all, so the heatsink shown is definitely overkill. However, it still helps to have an efficient heatsink to help prevent the LM3886 from activating its internal SPike protection circuitry - which has been blamed for a lot of audible artifacts.

 

[linuxguru]

There are these large caps at the signal input's. Anyone know what they are for?

The Twisted Pear boards don't have space for premium DC-blocking caps on-board. These (e.g. Obbligato, Mundorf, Solen, etc.) tend to be larger than the regular types. Hence it has to be kept outboard, near the connector.

 

[keith_correa]

The Twisted Pear boards don't have space for premium DC-blocking caps on-board. These (e.g. Obbligato, Mundorf, Solen, etc.) tend to be larger than the regular types. Hence it has to be kept outboard, near the connector.

Don't most [?] CDP's have blocking caps at their signal output? if they do, then there would be no need for these at the amp. signal inputs right?

 

[linuxguru]

Don't most [?] CDP's have blocking caps at their signal output? if they do, then there would be no need for these at the amp. signal inputs right?

You may omit the input DC-blocking cap if your signal source has a DC-blocking cap or the input DC offset voltage is so low that the output DC offset voltage is also low - say 50 mV or lower at the speaker terminals.

I measured it to be 6 mV and 15 mV, respectively at my two channels, and omitted the DC blocking caps. However, it will vary with different samples of the LM3886 and LM318, so this has to be measured on a case-by-case basis, and also depends on your signal source. A good CDP may have no output offset, but an el-cheapo computer sound card could have 100 mV or higher, which will translate to an output offset at the speaker terminals of 3V or higher - which will eventually fry the woofer.