DIY Audio Projects Forum

Hey Bruce.
I’m new to the game here and would love to do a build of you latest design.
Do you have a parts list for this latest version?

Thanks!

Hi, I don't, but there is probably someone on the site that might. If someone has a list can you please post it. The reason I don't have one is that I figure the project is at about the skill level for construction that anyone making one would be able to figure out the parts from the schematics. Plus when a particular component is recommended it shows in the schematic. The others are generic. Folks often have preferences for particular capacitors and such so I didn't want to limit their choices unnecessarily either.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

After quite a few years of enjoying my oddblocks, I'm taking them apart to reimpregnate a humming power transformer. At the same time I'd like to adjust the nfb. I have mine constantly disconnected as the sound was quite thin and harsh with the nfb connected. Probably because I'm using different iron/tubes. However, I would like it to have a touch more treble. Can you provide some advice on which nfb component value should I play with?

Best regards,

Paul

Hi, if the sound with nfb on is thin....then likely the phase is incorrect. A quick way to tell is if the phase is OK when connected is that the volume will be a little lower than when it is not connected.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Good day Bruce. I am wondering if I am to use the NFB, does the output of the transformer have to be grounded to the power supply ground in order to be effective?

Hi, Yes. It needs to have a complete path. If you don't the circuit will likely find another path that actually wanders back though the primary windings and really cause problems. If for some reason you can not use a grounded output then I would not use NFB. The amps behave well and the sound is excellent. The NFB is there mostly for above audio band control. Modeling indicates that with some speakers systems with really odd impedance issues the way above band resonance of the transformers could cause oscillation (at about 70K HZ) and mess up the sound and it would be difficult for a user to figure out what was happening. The amp would sound funny and be using a lot of power and not making a lot of sound. The NFB eliminates that possibility. I expect that the issue while possible is so remote that I nearly left out the NFB. I like to cover the bases though and put it in.

Be aware of the possibility of getting the phase backwards on the NFB. If it is, the sound will be thin. If you disconnect the NFB and it is correct the volume will increase slightly. If it is backwards the sound will get softer. There are several ways to fix it. The usual way is to swap the wires going to the anodes and screens from one output tube to the other. Another sometimes easier way is to switch the input grid connections from one output tune to the other (swap signal input and ground). This effectively changes the phase of the stage. If you only use the 8 ohm speaker connections you can swap the ground and 8 ohm ones.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Thanks for the detailed response Bruce! The NFB has been working fine for several years. While I was trying to track down a hum in my output, I removed the 150R resistor and capacitor from both mono blocks and only left one piece of gear with its connection to earth ground via a 150R and a capacitor in parallel. I accidentally cut the negative wire that connected the 0 transformer output to signal ground. I was just trying to rule out ground loops between equipment but it turns out that most, if not all the hum is from induction. The power supply transformer for the Groovewatt is separated from the rest of its chassis and in a metal enclosure that is also grounded to earth. Well, the inputs from the turntable are about 2 inches away from the box that contains the transformer. There is enough magnetic flux radiating from the box that it's getting into the input wires and eventually amplified. I only discovered this by trying (at wits end) a small RCA isolation transformer. When I was moving the little RCA transformer around near the inputs to the Groovewatt, I could either make it worse (louder) or much more quieter if I moved it away.

Installed new Canare RCA cables and installed a new shield on the tonearm wires and 99% of my hum is gone. Can very faintly hear a hum with the Groove watt and the Oddblocks and the preamp turned up to Max.

Hi, I caution about removing the 150R and X2 capacitor. The case will not have a clear path to the earth ground. This can defeat its role in fault protection as well as EMI shielding. Having just one pair in a different piece of gear is not likely to act as fault protection. You can replace the 150R and X2 with a bridge rectifier as is often done in guitar gear and it will handle the faults fine, but will still not entirely remove low level EMI. Diodes after all have a finite voltage minimum conduction point. If the R/X2 is causing hum then you probably have a loop involving the cables or a piece of gear that uses the chassis as signal ground. Yes having the turntable near a power source is almost certainly going to cause hum. The SUTs I use for LOMC cartridges can sniff out hum from many inches away. Even with a steel enclosure and mu metal case.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

My apologies Bruce. I didn't mention that I left the earth ground connected on each piece of gear. I just removed one side of the 150R and capacitor that ties the earth ground to the signal ground. I left the preamp connection to earth ground connected because everything connects to the preamp.

Hello everyone (I'm new here),

Recently built a pair of KT120 monoblocks based on Bruce's Oddblocks and the first block sounds amazing! Unfortunately the second one is having an issue that I just can't seem to pin down.

I have designed circuit boards for mine and have assembled both units in the exact same way (pictures attached), but for some reason the second block's relay keeps resetting after the heater delay. The relay clicks, but immediately disengages. The output pin on the 555 seems to stay high at 13Vdc and the delay starts over again. I've checked the 555 timer and the relay using a breadboard and they are working fine. I noticed when using a multi meter to check the voltage by chance at pin 5 on the 555, the output pin will go low and the relay engages.... I get power to the high voltage side and better yet, the amp works. If I power the unit down and try powering up again, I have the same issue. If I pull the tubes and power it up, the relay works. Also, if I leave the tubes in, but disconnect the 360Vac the relay also works . I suspect something in the heater bias is causing the 555 timer to reset as soon as the relay engages, but I can't seem to identify the cause. All voltages, resistors and caps seem to check out when compared to the working unit. Maybe an issue with the LR8?

Any thoughts or insights would be greatly appreciated.

Best Regards,

Jeff