Finally assembled, but not sounding great

[vakman]

Hello fellow R2R enthusiasts,

I ordered my Eros Tape kit last December, but finally had it finished yesterday.

It all seems to work fine and has been carefully assembled.

Only issue is the sound. It’s lacking highs, Bass and sounds a little muffled. Definitely worse than using stock inbuilt preamp.

Unit is a Technics RS1500 with the Eros connected to the tape head in a clever way. Using existing head cables, but connected to a new switchable RCA board at the back of the Technics.

Any suggestions on where to start?

Appreciate any help.

Thanks

 

[Paul Birkeland]

How did the voltage check go?

 

[Doc B]

Typically if this is a dramatic difference it could be due to an error in the wiring of the eq components, or possibly something in the switching setup could be causing a problem. Does the switch completely switch out the internal preamp when switching to the Eros? If the head is being loaded by both preamps at the same time that could cause a problem. If the difference is subtle it can be something as simple as cable capacitance from the deck to the Eros input, or from the Eros output to the equipment downstream of it impacting the sound. I have been to more than one trade show where we needed to change the cables from deck to Eros or Eros to preamp because of a cable mismatch.

Are the voltages all correct in the Eros? Have you run a tone tape to see what the frequency response of the deck is like when using the Eros?

 

[vakman]

I'm not sure, the person who serviced by Technics built the pre-amp at my request.

I suppose, once I open it up and take a photos and post them here, we should be able to find a solution to the problem.

If it helps, I do have a good soldering iron, multi-meter, oscilloscope + an alignment tape.

I will try and seek guidance on what to do here. Hope that's ok with everyone.

Thanks again

 

[Deke609]

If it helps, I do have a... multi-meter

Do the resistance and voltage checks set out in the manual and post any discrepancies.

cheers, Derek

 

[vakman]

Hi All,

I've measured the voltages and they all seem in line with normal expectations

Left
1A = 228
0B = 93.3
bA = 0
0A = 173.2
Breg = 95
Kreg = 1.469

Right
1A = 224.5
0B = 96.8
bA = 0
0A = 166.7
Breg = 97.9
Kreg = 1.696

Also sharing a close up view of the circuit board for review

 

[vakman]

Here is the photo. Finding it hard to post the photo. Keeps timing out

 

[vakman]

Some additional images of the Technics, with the new tiny board and mechanism to allow the new pre-amp to be connected to the heads with the original wiring.

 

[vakman]

Typically if this is a dramatic difference it could be due to an error in the wiring of the eq components, or possibly something in the switching setup could be causing a problem. Does the switch completely switch out the internal preamp when switching to the Eros? If the head is being loaded by both preamps at the same time that could cause a problem. If the difference is subtle it can be something as simple as cable capacitance from the deck to the Eros input, or from the Eros output to the equipment downstream of it impacting the sound. I have been to more than one trade show where we needed to change the cables from deck to Eros or Eros to preamp because of a cable mismatch.

Are the voltages all correct in the Eros? Have you run a tone tape to see what the frequency response of the deck is like when using the Eros?

Yes, switching is completely independent and not coupled together. eg. Internal is completely separate from External, as can be shown in the photos and connected to the main Technics board using an identical plug

Not sure how I can measure the cable capacitance from the deck to the Eros input

 

[Doc B]

It's not super clear from the images but I am guessing that you pull the gray repro head cable plug from where it is plugged in to the internal preamp as shown in the photos, and re-plug it into the little header on the RCA jack board, which feeds the RCA jacks closest to the header to create a direct head output. Is that correct?

That looks like it should work. I would suggest running a tone tape and determining what the frequency response is like. You can use a Y adapter at the output of the Eros and feed the signal from the extra output of the y adapter back to the line input of the deck. That way you can monitor the line in signal on the Technics meters. This would help determine of the issue is between the head and the Eros or the Eros and the rest of the system.

Do you know the input impedance of the preamp that the Eros output is connected to? It is best if it is above 15K ohms, and short cables, like 1M or less are best. Interconnect cable capacitance is usually spec'd by the manufacturer or if you have something DIY you can usually look up the spec of the cable used. It is spec'd by the foot, ideally something low like under 20pF/ft. You want the lowest capacitance possible from head to Eros, and low capacitance from Eros to preamp as well. If you DIY cables, there are several foamed teflon coax cables that are around 17pF/ft that work well.

 

[vakman]

It's not super clear from the images but I am guessing that you pull the gray repro head cable plug from where it is plugged in to the internal preamp as shown in the photos, and re-plug it into the little header on the RCA jack board, which feeds the RCA jacks closest to the header to create a direct head output. Is that correct?

That looks like it should work. I would suggest running a tone tape and determining what the frequency response is like. You can use a Y adapter at the output of the Eros and feed the signal from the extra output of the y adapter back to the line input of the deck. That way you can monitor the line in signal on the Technics meters. This would help determine of the issue is between the head and the Eros or the Eros and the rest of the system.

Do you know the input impedance of the preamp that the Eros output is connected to? It is best if it is above 15K ohms, and short cables, like 1M or less are best. Interconnect cable capacitance is usually spec'd by the manufacturer or if you have something DIY you can usually look up the spec of the cable used. It is spec'd by the foot, ideally something low like under 20pF/ft. You want the lowest capacitance possible from head to Eros, and low capacitance from Eros to preamp as well. If you DIY cables, there are several foamed teflon coax cables that are around 17pF/ft that work well.

Thanks Dan, I'll try and give it a go by running a tone tape (if I can find one) and will try and pick up a set of low capacitance cables over the next few days.

But otherwise, guessing the Eros has been built correctly, if all the voltages are in line with specifications. I mean is there a chance the resistors are not correct, as my construction manual for the Eros is dated October 6, 2013.

I've included a slightly better resolution photo of the circuit board if that helps.

 

[vakman]

Ok, I think it’s fixed. Just needed to flip a switch on the main preamp to increase the gain. Not sure why, as I would have thought the Eros had more gain than the inbuilt Technics preamp.

Now, because I increased the gain, now have a very noticeable hum coming from the speakers

 

[Paul Birkeland]

I would recommend taking a smart phone and playing a 60Hz tone into the Repro with your phone's volume set to the lowest setting above mute, then use your meter to measure the AC voltage coming out of the output jacks.